diff --git a/addon/content/pEpMimeEncrypt.js b/addon/content/pEpMimeEncrypt.js
index 831568d..c56da6a 100644
--- a/addon/content/pEpMimeEncrypt.js
+++ b/addon/content/pEpMimeEncrypt.js
@@ -136,8 +136,11 @@ MimeEncrypt.prototype = {
       this.isDraftOrTemplate);
     let encryptedMessage = this.controller.synchronise(encryptedPromise);
 
+    // The pEp engine assigns a new message ID to the encrypted message, but Thunderbird
+    // ignores that. So `encryptedMessage.id = message.id;` has no effect here.
+    // The desired message ID is already in the inner message.
+
     // If it wasn't encrypted after all (ENC_FORMAT_NONE), get the source, otherwise the destination.
-    encryptedMessage.id = message.id;  // Use the same ID, so we can access the message from the cache.
     let mimePromise = this.controller.cache_mime_encode_message(encryptedMessage.enc_format == ENC_FORMAT_NONE ? 0 : 1,
       encryptedMessage, true);
     this.outBuffer = this.controller.synchronise(mimePromise);