Are you sure your Exchange server had actually been hacked, and wasn't just relaying? That happened to me too, ages ago, but I wouldn't call it a hack - just a

Are you sure your Exchange server had actually been hacked, and wasn't just relaying? That happened to me too, ages ago, but I wouldn't call it a hack - just a bad misconfiguration, got me blacklisted at the time. In fact it was so long ago I can't even remember if it was called Exchange or something else; this is NT 3.51. Ah, NT, how I don't miss thee. Although, to be fair, I hear the MS toolchain has improved greatly in recent years.