Hi @Dan Northrop ,
Welcome to our forum!
According to your description, seems like it's related to a transient backend incident. If you are the administrator, you might be able to see the EX637339 in the Issue history of the Service Health Dashboard:
As mentioned in the above, please verify if you’ve encountered this problem in Outlook for Mac by reviewing their Sent Items to validate if a sent message contains an attachment. If you don't see the attachment, you can reply to the message in their Sent Items folder to reattach and send the message again.
Please be patient and stay tuned for the update.
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