Hello Blaine Rhoden,
Good day!
Thank you for posting to Microsoft Community. We are happy to help you.
To narrow down your issue, I will need the information below and give you the possible answer on this issue:
-Please can you run the extended message trace . For your reference : Message trace in the modern EAC in Exchange Online | Microsoft Docs and share with us the csv file via the private message
To run a message trace, you need to be a member of one of the following role groups:
- Global Administrator
- Exchange Administrator
-Does it happen for all message with attachment
The information above will allow us to provide the right answer to you,
we look forward to hearing from you soon
Sincerely,
Eben Ezer Tres | Microsoft Community Moderator