Exchange SE HU6: PDF attachments truncated to 13 KB via Outlook Desktop — OWA unaffected

Hi, the OWA-versus-Outlook difference is the key clue here. If OWA sends the same PDF correctly but Outlook desktop truncates it, I would test the Outlook submission path before assuming transport is the only layer involved.

I would try:

1. Send the same PDF from Outlook in Online Mode and Cached Mode.

2. Test a new Outlook profile on a clean workstation.

3. Compare MAPI submission versus OWA submission in message tracking.

4. Temporarily test with add-ins disabled in Outlook.

5. Check whether the issue happens with every PDF or only PDFs from a specific generator/scanner.

Since you already ruled out a lot of server-side filtering, I would collect one clean repro with message tracking, Outlook version/build, and the original/sent attachment sizes, then take that to Microsoft Support.