I've attached part of the full header to this post, there are no other similar addresses on it, just the one I created for this forum, which I have never used to send an email, nor have I posted it anywhere other than the registration form for this forum.
Hopefully it is just a random fluke coincidence, otherwise there is a leak of data somewhere.
[ BTW the email was apparently from a revolutionary political organisation, so they're probably not above doing something illegal to further their cause, e.g. hacking databases ]