Instead of adding a header, you could generate email messages with
a unique Message-ID: header (compliant with RFC 5322, of course).
That header identifies the message even as it travels from one MTA
to the next, as it is returned in (non)delivery notifications, and
so on.
Additionally, a Postfix MTA assigns each message a queue identifier
(queueid) that is unique for its mail queue, as long as you configure
Postfix with "enable_long_queue_ids = yes", and as long as your
system clock (UTC) does not jump back. This queueid is logged along
with routine MTA activity, so it can be useful for local tracking.
Wietse
