diff --git a/src/main/java/de/assecutor/votianlt/controller/MessageApiController.java b/src/main/java/de/assecutor/votianlt/controller/MessageApiController.java
index 260a651..1719bf8 100644
--- a/src/main/java/de/assecutor/votianlt/controller/MessageApiController.java
+++ b/src/main/java/de/assecutor/votianlt/controller/MessageApiController.java
@@ -31,25 +31,23 @@ public class MessageApiController {
     /**
      * Send a general message to a client
      * POST /api/messages/send
-     * Body: { "content": "message text", "sender": "username", "receiver": "username", "contentType": "TEXT|IMAGE" }
+     * Body: { "content": "message text", "receiver": "appUserId", "contentType": "TEXT|IMAGE" }
      */
     @PostMapping("/send")
     public ResponseEntity<Message> sendGeneralMessage(@RequestBody Map<String, String> request) {
         try {
             String content = request.get("content");
-            String sender = request.get("sender");
             String receiver = request.get("receiver");
             MessageContentType contentType = resolveContentType(request.get("contentType"));
 
-            if (content == null || content.isBlank() || 
-                sender == null || sender.isBlank() || 
+            if (content == null || content.isBlank() ||
                 receiver == null || receiver.isBlank()) {
                 log.warn("Invalid message request: missing required fields");
                 return ResponseEntity.badRequest().build();
             }
 
-            Message message = messageService.sendGeneralMessageToClient(content, sender, receiver, contentType);
-            log.info("General message sent from {} to {}", sender, receiver);
+            Message message = messageService.sendGeneralMessageToClient(content, receiver, contentType);
+            log.info("General message sent to AppUser '{}'", receiver);
             return ResponseEntity.ok(message);
 
         } catch (IllegalArgumentException e) {
@@ -64,21 +62,19 @@ public class MessageApiController {
     /**
      * Send a job-related message to a client
      * POST /api/messages/send-job-message
-     * Body: { "content": "message text", "sender": "username", "receiver": "username", 
+     * Body: { "content": "message text", "receiver": "appUserId",
      *         "jobId": "job id", "jobNumber": "job number", "contentType": "TEXT|IMAGE" }
      */
     @PostMapping("/send-job-message")
     public ResponseEntity<Message> sendJobMessage(@RequestBody Map<String, String> request) {
         try {
             String content = request.get("content");
-            String sender = request.get("sender");
             String receiver = request.get("receiver");
             String jobIdStr = request.get("jobId");
             String jobNumber = request.get("jobNumber");
             MessageContentType contentType = resolveContentType(request.get("contentType"));
 
-            if (content == null || content.isBlank() || 
-                sender == null || sender.isBlank() || 
+            if (content == null || content.isBlank() ||
                 receiver == null || receiver.isBlank() ||
                 jobIdStr == null || jobIdStr.isBlank()) {
                 log.warn("Invalid job message request: missing required fields");
@@ -86,8 +82,8 @@ public class MessageApiController {
             }
 
             ObjectId jobId = new ObjectId(jobIdStr);
-            Message message = messageService.sendJobMessageToClient(content, sender, receiver, contentType, jobId, jobNumber);
-            log.info("Job-related message sent from {} to {} for job {}", sender, receiver, jobNumber);
+            Message message = messageService.sendJobMessageToClient(content, receiver, contentType, jobId, jobNumber);
+            log.info("Job-related message sent to AppUser '{}' for job {}", receiver, jobNumber);
             return ResponseEntity.ok(message);
 
         } catch (IllegalArgumentException e) {
diff --git a/src/main/java/de/assecutor/votianlt/controller/MessageController.java b/src/main/java/de/assecutor/votianlt/controller/MessageController.java
index e402510..ded5f69 100644
--- a/src/main/java/de/assecutor/votianlt/controller/MessageController.java
+++ b/src/main/java/de/assecutor/votianlt/controller/MessageController.java
@@ -613,27 +613,19 @@ public class MessageController {
      * Handle incoming message from a client via MQTT.
      * Client sends to /server/{clientId}/message with payload:
      * {
-     *   "sender": "appUserUsername",
-     *   "receiver": "systemUserUsername",
      *   "content": "message payload",
      *   "contentType": "TEXT|IMAGE",
      *   "jobId": "optional job id",
-     *   "jobNumber": "optional job number",
-     *   "clientId": "extracted from topic"
+     *   "jobNumber": "optional job number"
      * }
      *
-     * Logic:
-     * 1. Extract clientId from topic (this is the AppUser ID)
-     * 2. Find AppUser by ID in database
-     * 3. Get owner (User) from AppUser.owner field
-     * 4. Set receiver = User ID, sender = AppUser ID
+     * The clientId is extracted from the MQTT topic and represents the AppUser ID.
+     * This clientId is stored as the receiver field in the message.
      */
     public void handleIncomingMessage(Map<String, Object> payload) {
         log.info("MQTT Endpoint '/server/{clientId}/message' called with data: {}", payload);
 
         try {
-            ChatMessageInboundPayload inboundPayload = ChatMessageInboundPayload.fromPayload(payload);
-
             // Extract clientId from payload (added by MqttV5ClientManager from topic)
             // The clientId IS the AppUser ID
             String clientId = payload.get("clientId") != null ? payload.get("clientId").toString() : null;
@@ -643,53 +635,15 @@ public class MessageController {
                 return;
             }
 
-            // Convert clientId (AppUser ID) to ObjectId
-            ObjectId appUserObjectId;
-            try {
-                appUserObjectId = new ObjectId(clientId);
-            } catch (IllegalArgumentException e) {
-                log.warn("Invalid clientId/AppUser ID '{}': {}", clientId, e.getMessage());
-                return;
-            }
+            // Add clientId as receiver to the payload
+            payload.put("receiver", clientId);
 
-            // Find AppUser by ID
-            AppUser appUser = appUserService.findById(appUserObjectId);
-            if (appUser == null) {
-                log.warn("AppUser not found for clientId '{}'", clientId);
-                return;
-            }
+            // Parse the payload
+            ChatMessageInboundPayload inboundPayload = ChatMessageInboundPayload.fromPayload(payload);
 
-            // Get owner (User) of AppUser from the owner field
-            ObjectId ownerId = appUser.getOwner();
-            if (ownerId == null) {
-                log.warn("AppUser '{}' has no owner, cannot determine receiver", clientId);
-                return;
-            }
-
-            // Verify that owner exists
-            de.assecutor.votianlt.model.User owner = userService.findById(ownerId);
-            if (owner == null) {
-                log.warn("Owner User not found for AppUser '{}'", clientId);
-                return;
-            }
-
-            // Convert owner ID to string for receiver field
-            String ownerIdString = ownerId.toHexString();
-
-            // Create payload with:
-            // - sender = AppUser ID (clientId)
-            // - receiver = User ID (owner's ID as string)
-            ChatMessageInboundPayload resolvedPayload = new ChatMessageInboundPayload(
-                clientId,       // sender = AppUser ID
-                ownerIdString,  // receiver = User ID
-                inboundPayload.content(),
-                inboundPayload.contentType(),
-                inboundPayload.jobId(),
-                inboundPayload.jobNumber()
-            );
-
-            messageService.receiveMessageFromClient(resolvedPayload);
-            log.info("Successfully saved incoming message from AppUser '{}' to User '{}'", clientId, ownerIdString);
+            // Save the message with receiver = AppUser ID (clientId)
+            messageService.receiveMessageFromClient(inboundPayload);
+            log.info("Successfully saved incoming message for AppUser '{}'", clientId);
         } catch (IllegalArgumentException validationError) {
             log.warn("Incoming chat message rejected: {}", validationError.getMessage());
         } catch (Exception e) {
diff --git a/src/main/java/de/assecutor/votianlt/dto/ChatMessageInboundPayload.java b/src/main/java/de/assecutor/votianlt/dto/ChatMessageInboundPayload.java
index 6225396..36dee77 100644
--- a/src/main/java/de/assecutor/votianlt/dto/ChatMessageInboundPayload.java
+++ b/src/main/java/de/assecutor/votianlt/dto/ChatMessageInboundPayload.java
@@ -6,8 +6,9 @@ import org.bson.types.ObjectId;
 
 /**
  * Normalized payload for chat messages sent by mobile clients via MQTT.
+ * receiver = AppUser ID (clientId) extracted from MQTT topic
  */
-public record ChatMessageInboundPayload(String sender, String receiver, String content,
+public record ChatMessageInboundPayload(String receiver, String content,
         MessageContentType contentType, ObjectId jobId, String jobNumber) {
 
     public ChatMessageInboundPayload {
@@ -19,14 +20,13 @@ public record ChatMessageInboundPayload(String sender, String receiver, String c
             throw new IllegalArgumentException("payload must not be null");
         }
 
-        String sender = extractRequiredString(payload, "sender");
         String receiver = extractRequiredString(payload, "receiver");
         String content = extractRequiredString(payload, "content");
         MessageContentType contentType = extractContentType(payload.get("contentType"));
         ObjectId jobId = extractObjectId(payload.get("jobId"), "jobId");
         String jobNumber = extractOptionalString(payload.get("jobNumber"));
 
-        return new ChatMessageInboundPayload(sender, receiver, content, contentType, jobId, jobNumber);
+        return new ChatMessageInboundPayload(receiver, content, contentType, jobId, jobNumber);
     }
 
     public boolean hasJobContext() {
diff --git a/src/main/java/de/assecutor/votianlt/dto/ChatMessageOutboundPayload.java b/src/main/java/de/assecutor/votianlt/dto/ChatMessageOutboundPayload.java
index f8cf67e..e23b85f 100644
--- a/src/main/java/de/assecutor/votianlt/dto/ChatMessageOutboundPayload.java
+++ b/src/main/java/de/assecutor/votianlt/dto/ChatMessageOutboundPayload.java
@@ -8,11 +8,10 @@ import java.time.LocalDateTime;
 
 /**
  * Outbound chat message payload published to MQTT subscribers.
+ * The receiver is implicit from the MQTT topic (/client/{appUserId}/message)
  */
 public record ChatMessageOutboundPayload(
         String messageId,
-        String sender,
-        String receiver,
         String content,
         MessageContentType contentType,
         MessageOrigin origin,
@@ -26,8 +25,6 @@ public record ChatMessageOutboundPayload(
     public static ChatMessageOutboundPayload fromMessage(Message message) {
         return new ChatMessageOutboundPayload(
                 message.getIdAsString(),
-                message.getSender(),
-                message.getReceiver(),
                 message.getContent(),
                 message.getContentType(),
                 message.getOrigin(),
diff --git a/src/main/java/de/assecutor/votianlt/model/Message.java b/src/main/java/de/assecutor/votianlt/model/Message.java
index e4aaede..6b5b5b4 100644
--- a/src/main/java/de/assecutor/votianlt/model/Message.java
+++ b/src/main/java/de/assecutor/votianlt/model/Message.java
@@ -37,13 +37,7 @@ public class Message {
     private MessageContentType contentType = MessageContentType.TEXT;
 
     /**
-     * Username of the sender (app user or system user)
-     */
-    @Field("sender")
-    private String sender;
-
-    /**
-     * Username of the receiver (app user or system user)
+     * AppUser ID (clientId) - the AppUser to whom this message belongs
      */
     @Field("receiver")
     private String receiver;
@@ -93,33 +87,33 @@ public class Message {
     /**
      * Constructor for general messages
      */
-    public Message(String content, String sender, String receiver, MessageOrigin origin) {
-        this(content, sender, receiver, origin, MessageContentType.TEXT);
+    public Message(String content, String receiver, MessageOrigin origin) {
+        this(content, receiver, origin, MessageContentType.TEXT);
     }
 
     /**
      * Constructor for general messages with explicit content type
      */
-    public Message(String content, String sender, String receiver, MessageOrigin origin,
+    public Message(String content, String receiver, MessageOrigin origin,
             MessageContentType contentType) {
-        initializeBaseFields(content, sender, receiver, origin, contentType);
+        initializeBaseFields(content, receiver, origin, contentType);
         this.messageType = MessageType.GENERAL;
     }
 
     /**
      * Constructor for job-related messages
      */
-    public Message(String content, String sender, String receiver, MessageOrigin origin,
+    public Message(String content, String receiver, MessageOrigin origin,
             ObjectId jobId, String jobNumber) {
-        this(content, sender, receiver, origin, MessageContentType.TEXT, jobId, jobNumber);
+        this(content, receiver, origin, MessageContentType.TEXT, jobId, jobNumber);
     }
 
     /**
      * Constructor for job-related messages with explicit content type
      */
-    public Message(String content, String sender, String receiver, MessageOrigin origin,
+    public Message(String content, String receiver, MessageOrigin origin,
             MessageContentType contentType, ObjectId jobId, String jobNumber) {
-        initializeBaseFields(content, sender, receiver, origin, contentType);
+        initializeBaseFields(content, receiver, origin, contentType);
         this.messageType = MessageType.JOB_RELATED;
         this.jobId = jobId;
         this.jobNumber = jobNumber;
@@ -156,10 +150,9 @@ public class Message {
         return contentType != null ? contentType : MessageContentType.TEXT;
     }
 
-    private void initializeBaseFields(String content, String sender, String receiver, MessageOrigin origin,
+    private void initializeBaseFields(String content, String receiver, MessageOrigin origin,
             MessageContentType contentType) {
         this.content = content;
-        this.sender = sender;
         this.receiver = receiver;
         this.origin = origin;
         this.createdAt = LocalDateTime.now();
diff --git a/src/main/java/de/assecutor/votianlt/pages/base/ui/view/MainLayout.java b/src/main/java/de/assecutor/votianlt/pages/base/ui/view/MainLayout.java
index 1a721e5..c338d69 100644
--- a/src/main/java/de/assecutor/votianlt/pages/base/ui/view/MainLayout.java
+++ b/src/main/java/de/assecutor/votianlt/pages/base/ui/view/MainLayout.java
@@ -25,6 +25,7 @@ import com.vaadin.flow.server.menu.MenuEntry;
 import com.vaadin.flow.shared.Registration;
 import de.assecutor.votianlt.model.User;
 import de.assecutor.votianlt.model.UserInvoiceData;
+import de.assecutor.votianlt.pages.service.AppUserService;
 import de.assecutor.votianlt.pages.service.UserInvoiceDataService;
 import de.assecutor.votianlt.pages.view.EditProfileView;
 import de.assecutor.votianlt.security.SecurityService;
@@ -34,9 +35,8 @@ import lombok.extern.slf4j.Slf4j;
 
 import static com.vaadin.flow.theme.lumo.LumoUtility.*;
 
-import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Optional;
-import java.util.Set;
 
 @AnonymousAllowed
 @Slf4j
@@ -47,6 +47,7 @@ public final class MainLayout extends AppLayout {
     private final UserInvoiceDataService userInvoiceDataService;
     private final MessageService messageService;
     private final MessageBadgeUpdateService messageBadgeUpdateService;
+    private final AppUserService appUserService;
     private Div headerRef;
     private Scroller navRef;
     private Component userMenuRef;
@@ -55,11 +56,13 @@ public final class MainLayout extends AppLayout {
     private Registration badgeUpdateRegistration; // Track badge update listener registration
 
     public MainLayout(SecurityService securityService, UserInvoiceDataService userInvoiceDataService,
-            MessageService messageService, MessageBadgeUpdateService messageBadgeUpdateService) {
+            MessageService messageService, MessageBadgeUpdateService messageBadgeUpdateService,
+            AppUserService appUserService) {
         this.securityService = securityService;
         this.userInvoiceDataService = userInvoiceDataService;
         this.messageService = messageService;
         this.messageBadgeUpdateService = messageBadgeUpdateService;
+        this.appUserService = appUserService;
         setPrimarySection(Section.DRAWER);
 
         // Always build the drawer; keep references and toggle visibility on attach and
@@ -218,47 +221,28 @@ public final class MainLayout extends AppLayout {
             return 0;
         }
 
-        Set<String> candidateReceivers = new LinkedHashSet<>();
-
         try {
-            User currentUser = securityService.getCurrentDatabaseUser();
+            // Get all AppUsers for the current user
+            List<de.assecutor.votianlt.model.AppUser> appUsers = appUserService.findByCurrentUser();
 
-            if (currentUser != null) {
-                // Add User ID (ObjectId as string) - this is now the primary receiver identifier
-                if (currentUser.getId() != null) {
-                    candidateReceivers.add(currentUser.getId().toHexString());
-                }
+            if (appUsers == null || appUsers.isEmpty()) {
+                return 0;
+            }
 
-                // Also add email for backward compatibility with old messages
-                String email = Optional.ofNullable(currentUser.getEmail()).map(String::trim).orElse("");
-                if (!email.isBlank()) {
-                    candidateReceivers.add(email);
-                }
-
-                // Also add full name for backward compatibility
-                String fullName = ((Optional.ofNullable(currentUser.getFirstname()).orElse("") + " "
-                        + Optional.ofNullable(currentUser.getName()).orElse(""))).trim();
-                if (!fullName.isBlank()) {
-                    candidateReceivers.add(fullName);
+            // Count unread messages for all AppUsers (receiver = AppUser ID)
+            long unread = 0;
+            for (de.assecutor.votianlt.model.AppUser appUser : appUsers) {
+                if (appUser != null && appUser.getId() != null) {
+                    String appUserId = appUser.getId().toHexString();
+                    unread += messageService.getUnreadMessageCount(appUserId);
                 }
             }
-        } catch (RuntimeException ignored) {
-            // Fallback to username only
-        }
 
-        Optional.ofNullable(securityService.getCurrentUsername()).map(String::trim).filter(name -> !name.isBlank())
-                .ifPresent(candidateReceivers::add);
-
-        if (candidateReceivers.isEmpty()) {
+            return unread;
+        } catch (RuntimeException e) {
+            log.error("Error resolving unread message count: {}", e.getMessage(), e);
             return 0;
         }
-
-        long unread = 0;
-        for (String receiver : candidateReceivers) {
-            unread += messageService.getUnreadMessageCount(receiver);
-        }
-
-        return unread;
     }
 
     private Component createUserMenu() {
diff --git a/src/main/java/de/assecutor/votianlt/pages/view/MessageDetailsView.java b/src/main/java/de/assecutor/votianlt/pages/view/MessageDetailsView.java
index 923d751..168be7d 100644
--- a/src/main/java/de/assecutor/votianlt/pages/view/MessageDetailsView.java
+++ b/src/main/java/de/assecutor/votianlt/pages/view/MessageDetailsView.java
@@ -151,7 +151,7 @@ public class MessageDetailsView extends Main implements BeforeEnterObserver {
             clientName = Optional.ofNullable(client).map(AppUser::getBezeichnung).orElse("Unbekannter Teilnehmer");
         }
 
-        List<Message> allMessages = messageService.getMessagesForParticipantAscending(participantKey);
+        List<Message> allMessages = messageService.getMessagesForAppUserAscending(participantKey);
         List<Message> filteredMessages = filterMessagesForConversation(allMessages, conversationId);
 
         this.jobConversation = conversationId != null && conversationId.startsWith("job-");
@@ -808,10 +808,12 @@ public class MessageDetailsView extends Main implements BeforeEnterObserver {
         try {
             Message saved;
             if (jobConversation) {
-                saved = messageService.sendJobMessageToClient(payload, sender, participantKey,
+                // participantKey = AppUser ID (receiver)
+                saved = messageService.sendJobMessageToClient(payload, participantKey,
                         contentType, jobIdContext, jobNumberContext);
             } else {
-                saved = messageService.sendGeneralMessageToClient(payload, sender, participantKey,
+                // participantKey = AppUser ID (receiver)
+                saved = messageService.sendGeneralMessageToClient(payload, participantKey,
                         contentType);
             }
 
@@ -1018,12 +1020,11 @@ public class MessageDetailsView extends Main implements BeforeEnterObserver {
             return;
         }
 
-        // Check if message involves the current participant
-        boolean involvesParticipant = participantKey.equals(message.getSender())
-                                    || participantKey.equals(message.getReceiver());
+        // Check if message belongs to the current participant (receiver = AppUser ID)
+        boolean involvesParticipant = participantKey.equals(message.getReceiver());
 
         if (!involvesParticipant) {
-            log.debug("Message does not involve current participant, ignoring");
+            log.debug("Message does not belong to current participant, ignoring");
             return;
         }
 
diff --git a/src/main/java/de/assecutor/votianlt/pages/view/MessagesView.java b/src/main/java/de/assecutor/votianlt/pages/view/MessagesView.java
index 0b94a1d..b095119 100644
--- a/src/main/java/de/assecutor/votianlt/pages/view/MessagesView.java
+++ b/src/main/java/de/assecutor/votianlt/pages/view/MessagesView.java
@@ -258,9 +258,7 @@ public class MessagesView extends Main {
         if (message == null) {
             return null;
         }
-        if (message.getOrigin() == MessageOrigin.CLIENT) {
-            return message.getSender();
-        }
+        // All messages now have receiver = AppUser ID (clientId)
         return message.getReceiver();
     }
 
diff --git a/src/main/java/de/assecutor/votianlt/pages/view/UserMessagesView.java b/src/main/java/de/assecutor/votianlt/pages/view/UserMessagesView.java
index 69b0dd9..b89a124 100644
--- a/src/main/java/de/assecutor/votianlt/pages/view/UserMessagesView.java
+++ b/src/main/java/de/assecutor/votianlt/pages/view/UserMessagesView.java
@@ -85,7 +85,7 @@ public class UserMessagesView extends Main implements HasUrlParameter<String> {
         HorizontalLayout headerLayout = createHeaderLayout(clientName);
         contentLayout.add(headerLayout);
 
-        List<Message> conversation = messageService.getMessagesForParticipantAscending(participantKey);
+        List<Message> conversation = messageService.getMessagesForAppUserAscending(participantKey);
         Map<MessageType, List<Message>> messagesByType = conversation.stream()
                 .collect(Collectors.groupingBy(message -> Optional.ofNullable(message.getMessageType()).orElse(MessageType.GENERAL)));
 
diff --git a/src/main/java/de/assecutor/votianlt/repository/MessageRepository.java b/src/main/java/de/assecutor/votianlt/repository/MessageRepository.java
index f7f173b..cd3d5d0 100644
--- a/src/main/java/de/assecutor/votianlt/repository/MessageRepository.java
+++ b/src/main/java/de/assecutor/votianlt/repository/MessageRepository.java
@@ -13,17 +13,17 @@ import java.util.List;
 public interface MessageRepository extends MongoRepository<Message, ObjectId> {
 
     /**
-     * Find all messages for a specific receiver, ordered by creation time descending (newest first)
+     * Find all messages for a specific receiver (AppUser ID), ordered by creation time ascending (oldest first)
+     */
+    List<Message> findByReceiverOrderByCreatedAtAsc(String receiver);
+
+    /**
+     * Find all messages for a specific receiver (AppUser ID), ordered by creation time descending (newest first)
      */
     List<Message> findByReceiverOrderByCreatedAtDesc(String receiver);
 
     /**
-     * Find all messages sent by a specific sender, ordered by creation time descending
-     */
-    List<Message> findBySenderOrderByCreatedAtDesc(String sender);
-
-    /**
-     * Find all unread messages for a specific receiver
+     * Find all unread messages for a specific receiver (AppUser ID)
      */
     List<Message> findByReceiverAndIsReadFalseOrderByCreatedAtDesc(String receiver);
 
@@ -33,27 +33,22 @@ public interface MessageRepository extends MongoRepository<Message, ObjectId> {
     List<Message> findByJobIdOrderByCreatedAtDesc(ObjectId jobId);
 
     /**
-     * Find all messages of a specific type for a receiver
+     * Find all messages of a specific type for a receiver (AppUser ID)
      */
     List<Message> findByReceiverAndMessageTypeOrderByCreatedAtDesc(String receiver, MessageType messageType);
 
     /**
-     * Find all messages by origin (incoming/outgoing/server)
+     * Find all messages by origin (CLIENT/SERVER)
      */
     List<Message> findByOriginOrderByCreatedAtDesc(MessageOrigin origin);
 
     /**
-     * Find all messages between two users (in both directions)
-     */
-    List<Message> findBySenderAndReceiverOrderByCreatedAtAsc(String sender, String receiver);
-
-    /**
-     * Find all unread messages count for a specific receiver
+     * Find all unread messages count for a specific receiver (AppUser ID)
      */
     long countByReceiverAndIsReadFalse(String receiver);
 
     /**
-     * Find all messages for a specific receiver and job
+     * Find all messages for a specific receiver (AppUser ID) and job
      */
     List<Message> findByReceiverAndJobIdOrderByCreatedAtDesc(String receiver, ObjectId jobId);
 
@@ -61,16 +56,4 @@ public interface MessageRepository extends MongoRepository<Message, ObjectId> {
      * Find all messages (for admin/overview), ordered by creation time descending
      */
     List<Message> findAllByOrderByCreatedAtDesc();
-
-    /**
-     * Find all messages where the sender or receiver matches the provided value,
-     * ordered by creation time ascending.
-     */
-    List<Message> findBySenderOrReceiverOrderByCreatedAtAsc(String sender, String receiver);
-
-    /**
-     * Find all messages where the sender or receiver matches the provided value,
-     * ordered by creation time descending.
-     */
-    List<Message> findBySenderOrReceiverOrderByCreatedAtDesc(String sender, String receiver);
 }
diff --git a/src/main/java/de/assecutor/votianlt/service/MessageBroadcaster.java b/src/main/java/de/assecutor/votianlt/service/MessageBroadcaster.java
index c0641e2..0d4680d 100644
--- a/src/main/java/de/assecutor/votianlt/service/MessageBroadcaster.java
+++ b/src/main/java/de/assecutor/votianlt/service/MessageBroadcaster.java
@@ -64,7 +64,8 @@ public class MessageBroadcaster {
     @EventListener
     public void onMessageReceived(MessageReceivedEvent event) {
         Message message = event.getMessage();
-        log.info("MessageBroadcaster received event for message from: {}", message.getSender());
+        log.info("MessageBroadcaster received event for message with origin {} for receiver {}",
+                message.getOrigin(), message.getReceiver());
         broadcast(message);
     }
 }
diff --git a/src/main/java/de/assecutor/votianlt/service/MessageService.java b/src/main/java/de/assecutor/votianlt/service/MessageService.java
index 7847db8..d649719 100644
--- a/src/main/java/de/assecutor/votianlt/service/MessageService.java
+++ b/src/main/java/de/assecutor/votianlt/service/MessageService.java
@@ -41,20 +41,22 @@ public class MessageService {
      * Save a message to the database
      */
     public Message saveMessage(Message message) {
-        log.info("Saving message from {} to {}", message.getSender(), message.getReceiver());
+        log.info("Saving message with origin {} for receiver {}", message.getOrigin(), message.getReceiver());
         return messageRepository.save(message);
     }
 
     /**
      * Send a general message to a client via MQTT
+     * @param content Message content
+     * @param receiver AppUser ID (clientId)
      */
-    public Message sendGeneralMessageToClient(String content, String sender, String receiver) {
-        return sendGeneralMessageToClient(content, sender, receiver, MessageContentType.TEXT);
+    public Message sendGeneralMessageToClient(String content, String receiver) {
+        return sendGeneralMessageToClient(content, receiver, MessageContentType.TEXT);
     }
 
-    public Message sendGeneralMessageToClient(String content, String sender, String receiver,
+    public Message sendGeneralMessageToClient(String content, String receiver,
             MessageContentType contentType) {
-        Message message = new Message(content, sender, receiver, MessageOrigin.SERVER, contentType);
+        Message message = new Message(content, receiver, MessageOrigin.SERVER, contentType);
         message = saveMessage(message);
         publishMessageToMqtt(message, receiver);
         return message;
@@ -62,16 +64,20 @@ public class MessageService {
 
     /**
      * Send a job-related message to a client via MQTT
+     * @param content Message content
+     * @param receiver AppUser ID (clientId)
+     * @param jobId Job ObjectId
+     * @param jobNumber Job number
      */
-    public Message sendJobMessageToClient(String content, String sender, String receiver,
+    public Message sendJobMessageToClient(String content, String receiver,
             ObjectId jobId, String jobNumber) {
-        return sendJobMessageToClient(content, sender, receiver, MessageContentType.TEXT, jobId, jobNumber);
+        return sendJobMessageToClient(content, receiver, MessageContentType.TEXT, jobId, jobNumber);
     }
 
-    public Message sendJobMessageToClient(String content, String sender, String receiver,
+    public Message sendJobMessageToClient(String content, String receiver,
             MessageContentType contentType, ObjectId jobId, String jobNumber) {
         JobContext context = resolveJobContext(jobId, jobNumber);
-        Message message = new Message(content, sender, receiver, MessageOrigin.SERVER, contentType,
+        Message message = new Message(content, receiver, MessageOrigin.SERVER, contentType,
                 context.jobId(), context.jobNumber());
         message = saveMessage(message);
         publishMessageToMqtt(message, receiver);
@@ -80,24 +86,28 @@ public class MessageService {
 
     /**
      * Handle incoming message from a client
+     * @param payload Inbound message payload where receiver = AppUser ID (clientId)
      */
     public Message receiveMessageFromClient(ChatMessageInboundPayload payload) {
         Message message;
         MessageContentType contentType = payload.contentType();
         if (payload.hasJobContext()) {
             JobContext context = resolveJobContext(payload.jobId(), payload.jobNumber());
-            message = new Message(payload.content(), payload.sender(), payload.receiver(),
+            // receiver = AppUser ID (clientId)
+            message = new Message(payload.content(), payload.receiver(),
                     MessageOrigin.CLIENT, contentType, context.jobId(), context.jobNumber());
         } else {
-            message = new Message(payload.content(), payload.sender(), payload.receiver(),
+            // receiver = AppUser ID (clientId)
+            message = new Message(payload.content(), payload.receiver(),
                     MessageOrigin.CLIENT, contentType);
         }
         message = saveMessage(message);
-        
+
         // Publish event to notify UI components about the new message
-        log.info("Publishing MessageReceivedEvent for message from {}", message.getSender());
+        log.info("Publishing MessageReceivedEvent for message with origin {} for receiver {}",
+                message.getOrigin(), message.getReceiver());
         eventPublisher.publishEvent(new MessageReceivedEvent(this, message));
-        
+
         return message;
     }
 
@@ -144,25 +154,25 @@ public class MessageService {
     }
 
     /**
-     * Get all messages a participant sent or received (oldest first) to reconstruct
-     * the conversation timeline.
+     * Get all messages for a specific AppUser (by receiver field), ordered by creation time ascending (oldest first)
+     * @param appUserId AppUser ID (clientId)
      */
-    public List<Message> getMessagesForParticipantAscending(String participant) {
-        if (participant == null || participant.isBlank()) {
+    public List<Message> getMessagesForAppUserAscending(String appUserId) {
+        if (appUserId == null || appUserId.isBlank()) {
             return Collections.emptyList();
         }
-        return messageRepository.findBySenderOrReceiverOrderByCreatedAtAsc(participant, participant);
+        return messageRepository.findByReceiverOrderByCreatedAtAsc(appUserId);
     }
 
     /**
-     * Get all messages a participant sent or received (newest first) for
-     * quick summaries.
+     * Get all messages for a specific AppUser (by receiver field), ordered by creation time descending
+     * @param appUserId AppUser ID (clientId)
      */
-    public List<Message> getMessagesForParticipantDescending(String participant) {
-        if (participant == null || participant.isBlank()) {
+    public List<Message> getMessagesForAppUserDescending(String appUserId) {
+        if (appUserId == null || appUserId.isBlank()) {
             return Collections.emptyList();
         }
-        return messageRepository.findBySenderOrReceiverOrderByCreatedAtDesc(participant, participant);
+        return messageRepository.findByReceiverOrderByCreatedAtDesc(appUserId);
     }
 
     /**