I get a lot of bounces, undeliverables and other server errors from (or to) freebie mailers like yahoo, hotmail, and sadly, my own.

the funniest thing is sending stuff to myself sometimes (2 accounts on the same service, different domain names, one with a "country" tag, one with .net - my own mailer "won't accept e-mail with its own domain name sent from a foreign server".

Some accounts receive some messages, some never.

At first glance, I don't think it's been hijacked. It's just a crappy account.