diff --git a/packages/iios-service/src/messaging/message.service.ts b/packages/iios-service/src/messaging/message.service.ts
index 0df9d97..954285f 100644
--- a/packages/iios-service/src/messaging/message.service.ts
+++ b/packages/iios-service/src/messaging/message.service.ts
@@ -20,6 +20,7 @@ export interface MessageDto {
   id: string;
   threadId: string;
   senderActorId: string;
+  senderId: string; // the sender's stable externalId (email/username) — reliable "is this mine?"
   senderName: string;
   content: string;
   contentRef?: string;
@@ -506,6 +507,7 @@ export class MessageService {
       id: interaction.id,
       threadId,
       senderActorId: interaction.actorId ?? '',
+      senderId: interaction.actor?.sourceHandle?.externalId ?? '',
       senderName: interaction.actor?.displayName ?? interaction.actor?.sourceHandle?.externalId ?? 'unknown',
       content: text?.bodyText ?? '',
       contentRef: file?.contentRef ?? undefined,