PDF files stuck in Outlook beeing in EXO

Hey guys,

a customer of us is having trouble during his Exchange online pilot. Hybrid is created, which is not done by us. The customer reports a strange issue. Sometimes scanned or forwarded PDF files are stuck in the Outlook outbox.

Outlook is showing error 0x80040115 and This file came from another computer and might be blocked to help protect this computer

Sometimes ipconfig /flushdns may help and for me this does not fit. Why is it related to PDF files?
This is valid for EXO users in Outlook beeing in the company network. Transmission in OWA or from Homeoffice works.

Any ideas?

Kind regards,
woelki