Probably due to successful phishing (see: Targeted phishing attack against Department of Computer Science) at least one of our accounts has been cracked and used for spamming. All mail delivery has now been stopped and mall queues will be cleared. Clearing of the queues may take several hours. 

Update: More than 3 million spam messages have now been removed from the outgoing queues of our mail system and legit mail is flowing normally again.