From personal experience, whether you check that you want further
mailings from MoveOn.org or not, they send them to you anytime you send
anything (petitions, letters, etc) from their website. They're also not
that great about taking you off when you complain (I have had to
complain 2-3 times per incident). For this reason, no matter how I feel
about the subject, I won't go through them anymore.
Hopefully one of their contacts is listening, because their mail policy
is really obnoxious.
Xspedius Communications Co.