I have a service that sends out emails and listens to replies. I need a way to reliably pair each incoming reply with the correct outgoing email. Is there any way to embed some key in the outgoing email that will persist, so that when the recipient clicks "Reply to" in his email client, his reply will retain this key?
So far, my best bet is "faking" the from address in the outgoing email, so that the username is an ID (e.g. [email protected]) and the reply comes to this email. However, the obvious downside is that I have the added configuration effort of forwarding all incoming email from those non-existent email addresses to the one real email a, e.g. [email protected].
Every email has a unique mail ID. If you didn't give it one when you sent it the next mailserver will do it. See http://en.wikipedia.org/wiki/Message-ID
The response has that message-id in the In-reply-to header. Example - see here.
Best reference is the RFC 5322 "Internet Message Format", section 3.6.4: https://www.rfc-editor.org/rfc/rfc5322#section-3.6.4
This depends on all parties involved doing the right thing, but for the most part they'll do so these days. Of course, some software or human could send a reply without correctly set in-reply-to header, but that's the exception. When I write a reply to an email and use "New Mail" instead of "Reply", for example, my email of course won't have the in-reply-to header.