Some users’ email with attachments sent from Outlook for Mac may be delivered without attachments

Title: Some users’ email with attachments sent from Outlook for Mac may be delivered without attachments

User impact: Users’ email with attachments sent from Outlook for Mac may be delivered without attachments.

More info: Users can verify if they’ve encountered this problem in Outlook for Mac by reviewing their Sent Items to validate if a sent message contains an attachment. If users don’t see the attachment, they can reply to the message in their Sent Items folder to reattach and send the message again.

Current status: While working to deploy the additional fix, we uncovered another cause for the issue, which is that Outlook for Mac is making an additional service call to update the Information Rights Management (IRM) template and that changes the compliance ID on the server side, which causes the change of attachment ID and may trigger the mismatch error. We removed the additional call, which we expect will further reduce the number of observed errors and prevent further impact. The fix for this is being validated and we’ll provide an estimated timeline for resolution should one become available.

Scope of impact: This issue is specific to a very small percentage of users’ email with attachments sent from Outlook for Mac.

Start time: Monday, July 10, 2023, at 8:08 PM UTC

Root cause: A conflict occurring with how our service detects attachment IDs before sending email in Outlook for Mac is resulting in the service removing attachments on sending email.

Next update by: Tuesday, December 5, 2023, at 10:00 PM UTC