I have the following problem: An external mail is sent to two users of our tenant. The mail has an attachment (when delivered) of 28 MB. One of the users (user A) receives the mail with the attachment normally, but for the other address (User B), the mail sender receives an error message, stating, that the attachment is too big. The received diagnostic mail states, that the attachment is ~40 MB. If the mail is sent ONLY to User A, it is normally received, if sent ONLY to User B, the same size error occurs. If User A forwards the mail to User B, the mail is normally received.

Both users have the same send/receive file limits in Exchange (~35 MB).
What could cause this odd behavior?

Is there anything I can do, apart from setting a bigger size for the received attachments for User B – which solves the problem?

Thanks for you answer!