We have a mailbox that is getting flooded with messages, looks like a bot is signing them up for accounts and they are getting tons of confirmation messages (1000+ per hr).  the inbound messages are not tripping any RBLs or spam filters (we run commtouch and messagesniffer) so the vast majority go to the inbox.  any ideas on what to do when this happens?  I imagine they got compromised somewhere else and the flood is to cover up activity, but are there any suggestions on helping here or basically just need to ride it out?