Following the update of Symantec Messaging Gateway (SMG) to version 10.0.0 or greater SMG begins inserting a Message-ID header into email messages that did not originally have one. This is a departure from previous releases which did not add the missing Message-ID header.
After scanning email messages that do not contain a Message-ID header, the SMG appliance now inserts the missing Message-ID header prior to finaly delivery.
This is an expected change and intended to more closely align the SMG product behavior with email administrator expectations.
The Message-ID header added to messages which did not already contain one has the following form: