In contrast, once I've sent you a cookie, your MUA would include this
cookie in any Mail you send to me, so even this new message could be
automatically preprocessed by my MUA.
E.g. my customers do send fresh mails. The do not "reply" only. So
there is no useful MessageID contained in those messages.
If I understand correctly, you intend the cookie as a (semi-permanent)
instruction to the recipient's MUA to include the cookie in mails to your
address and/or your domain, in order to help "file" the response
efficiently at your end.
I can see some potential use in this, but wouldn't it be better to have a
method to instruct your local mail system to perform the same filing
actions on all email from that particular address? Particularly since the
recipient may change MUA?
Whether such instruction is valid as a goal of mail-ng or it should be
considered a "local configuration matter" I'm not sure.
Have I understood correctly? Would address filtering give you what you
expect to get from such a cookie?
jb