Messaging Gateway (SMG) is bouncing messages with more than 49 Received headers.
As messages are relayed by mail servers, each mail server inserts a "Received" header into the message headers to provide information on the route the message took during delivery. When more than 49 Received headers exist in the message it triggers the mail loop detection performed by SMG.
This is expected behavior.
The maximum number of Received headers allowed is not configurable.