A few days ago, I stumbled upon a server running qmail with qmail-scanner. The server was throwing out this error when a user on the server attempted to send an e-mail to someone else: 451 qq temporary problem (#4.3.0) The one thing I love about qmail is its extremely descriptive error messages. Did I say descriptive? I meant cryptic. Luckily, clamdscan was a bit more chatty in the general system logs: