*Update*

The spam filters have been cleared and internal service has been restored. There may still be some delays as external servers may have queued email as well but they should return to normal over the next few hours.
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Previous Description (2012-07-04 16:44:36):

Email delivery is delayed due to a large amount of SPAM messages passing through our mail filters.

ITS staff have identified the cause and are working to resolve the issue.