I'm half wondering if it's those self-appointed censors SpamHaus doing it, or something like it. I recently found I couldn't send mails to my own daughter because she's with Yahoo who were using that brick-built outhouse of a vetter, and the said bunch of kiteheads had identified my ISP as a source of spam and blacklisted them. Which is hardly urprising as they are 1&1, one of the world's biggest, hosting no end not only of their own mail, but mail from their customers systems. So, for the third time this year, we found our mail cut because of something beyond our control.
Given that 1&1's own spam filters uniquely manage to intercept real mail and misses all the junk, I think they're on a loser all round.