SpamAssassin applies a series of weighted tests to each incoming message to help detect the legitimacy of the message. Some tests will add points for known spam-like characteristics, others will subtract points if the message is more likely to be legitimate.
MUN uses a two stage system. Messages scoring 4.0 points or more are likely to be spam, though we recognize that this threshold is low enough to sometimes tag legitimate messages. That's why we mark the subject with xxSPAMxx and deliver it to your inbox where you may make your own judgements about the legitimacy of the message. Messages scoring more than 8.0 points are much more likely to be spam and are usually automatically discarded. About 80% of our total incoming mail is discarded by this process.
One spammer tactic is to use an incorrect future date. Since many people sort their inbox by date, this helps ensure that the spam message appears near the top of the list. SpamAssassin has a crude built-in rule, for trying to detect this: it considers any message with a year between 2010 and 2099 as far in the future. This rule added 3.4 points to the spam score.
With our 4.0 point threshold for xxSPAMxx, this rule penalized many legitimate messages we received starting at midnight on January 1. For example, a legitimate message with some minor spam-like characteristics that scored 0.8 points on December 31 would now score 4.2 and get tagged as xxSPAMxx.
Internal MUN email is automatically weighted with a high negative score, so this rule would not have adversely impacted any internal email messages.
SpamAssassin is widely used and many other email providers experienced the same problem. SpamAssassin issued a repair on January 1. We only became aware of the issue at Memorial on Monday, January 4. Once it came to our attention at about 3:30PM that day, the problem was quickly fixed.