Resolved as of 2008-03-13 09:30:00
To alleviate a particularly bad state of email delays yesterday we have started blocking hosts that appear on SpamHaus XBL and SBL blacklists in the morning. The effect was not immediate, as there were huge queues already. However, later that day we have seen the email delays drop from hours down to effectively 0 and this situation continued to stay this way
up until 7am today (when this mail is being sent).