When reviewing the Messaging Gateway (SMG) Message Audit Logs (MAL), some messages are undeliverable with the following response from the downstream mail mail server:
552 5.3.4 header size exceeds fixed maximum size
Product: Messaging Gateway
Component : MTA
This response is coming from the destination mail server and is usually due to the message exceeding the limit on total size of all message headers set in the destination mail server.
While SMG does insert a limited amount of metadata into the message headers when scanning email messages, this is usually not sufficient to cause the message headers to be too large unless they were already close to the upper limit set by the downstream mail server.
A mail routing loop in the email environment can result in this issue as the message accumulates Received headers and metadata as it repeatedly cycled through the mail routing loop.
If both the Accepted From and Delivery IP addresses are in your own network you will need to review the mail routeing configuration in both Messaging Gateway and the destination mail server to ensure that once mail has been scanned and delivered by Messaging Gateway it is not routed back to Messaging Gateway.