Exchange 2007: Bounces on nonexistent address when sending to address

I’m still trying to formulate a google search on this one. Strange thing happened this morning with one of our mailboxes. Sent an email to uname@domain.com (internal) and got a bounce message stating that “chillnoutname@domain.com” doesn’t exist. User still receives the original message. I can’t find this new address in AD or Exchange. I tried adding it as an alias but that didn’t stop the bounces.

This doesn’t seem like an infection, but I’m scanning anyway. Anyone else see something like this?

Thanks!