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Similar to pull request 2855 ( #2855 ), the message token will be inserted into MAIL FROM (i.e. for the return path for bounces).
In addition, there is a new configuration option (default disable the new feature), to look for this newly generated message tag when a bounce message comes in. If no tag is found, or the new lookup feature is disabled, then the old (backwards compatible) lookup for the X-Postal-MsgID header will be utilized to search for the corresponding referenced mail.
Using an individual mail address is way more stable than inserting a header into outgoing mails, and relaying on the existence of this header in incoming bounce messages. A lot of "real world" MTAs out there strip additional headers or compose new bounce mails not including the X-Postal-MsgID header. All those incoming bounces can be easily assigned to their corresponding outgoing mails by their individual message tag as part of the bounce mail address.