Some messages are not accepted by the internal Microsoft Exchange Server on the first deliver attempt. Reviewing the Message Audit Logs (MAL) shows that the message could not be delivered with the following message;
552 4.3.1 Session size exceeds fixed maximum session size
This response from the SMTP Transaction is due to the entire size of the SMTP Session itself which has reached the limit set on the remote receiver SMTP. By default, Messaging Gateway will not limit the total number of messages delivered over a single SMTP session to internal mail servers.
You can disable this limit or even extend the size that the Session Size from your Exchange, by following the instructions below:
How to Limit Inbound Session Size