I have checked transport rules and global and user limits are set for a very large attachments. Sending a 10MB pdf file yields the following:
This message was created automatically by mail delivery software.
A message that you have sent could not be delivered to one or more
recipients. This is a permanent error. The following address failed:
<firstname.lastname@example.org>: Backend Replied [70a7bc15.0.5645681.00-2147.8008874.p02c12m004.mxlogic.net]: 5.3.4 Message size exceeds fixed maximum message size (Mode: queuesafe)