Spam assault
I've had thousands of email correspondents over the years so on occasion a zombified machine or two used to send spam finds my domain in the compromised machine's address book and creates fake email addresses pointing back here. In those situations, I can get hundreds of "Undelivered mail return to sender" type messages whereever the spammer fails to find a proper address. Over the past couple of days I've been slammed with them (a hundred here, a hundred there) and in the past ten minutes I've gotten over 2000 and counting. They're filtered into the trash on my machine, but it's still crazy to see that big of a spam assault happen in something like real time.