Fixes #5941 - Block Microsoft Outlook Emoji Reactions in Zammad.

Co-authored-by: Rene Reimann <rr@zammad.com>

app/models/channel/email_build.rb

@@ -165,6 +165,10 @@ generate email with S/MIME
     mail['X-Mailer'] = 'Zammad Mail Service'
     # rubocop:enable Zammad/DetectTranslatableString
 
+    # Disallow reactions in Microsoft mail clients,
+    # see https://learn.microsoft.com/en-us/microsoft-365-apps/outlook/manage/manage-outlook-reactions.
+    mail['x-ms-reactions'] = 'disallow'
+
     mail
   end