I hate when that happens
Don’t you just love opening up Outlook over the weekend and discovering that some used your domain as a “from” address whilst spamming and now you’re left to deal with all the bounce-backs? There’s really nothing like watching the Inbox count go all the way up to almost 300, and then watch as SpamBayes identifies the bounces as spam and the Inbox count goes all the way back to zero. That’s a whole lot of processing power for nothing.
At least the from address wasn’t one I used, so I was able to go to the server and tell it to discard all mail sent to that address.