There is a recent incident EX637339.
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: We're continuing to develop an Outlook for Mac update that will correct this issue, that we still expect it will be available for download in mid-August. We anticipate providing a release date by our next communication update.
Scope of impact: This issue is specific to a very small percentage of users' email with attachments sent from Outlook for Mac.
Start time: Tuesday, July 11, 2023 at 4:08 AM GMT+8
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: Wednesday, August 16, 2023 at 2:30 AM GMT+