🚀 go-pugleaf

Your solution of appending a postmark on the message is a variant
of my method (2). This method should have been formulated in
more general terms, to append some kind of trace information
on the message. I prefer to put the postmark in the TO field
for the following reasons:

(a) What you want to stop is not the same message looping back
to the same host. This, in fact, can be perfectly legal in some
cases. What you want to stop is the message looping back to the
same mailing list.

(b) The TO: field is one of the safest field from being munged
by header-munging systems.

Best of course would be to have an additional, special field
in the headers intended for this kind of trace information -
provided that all systems accept and are willing to handle this
trace field in the proper manner.