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Why was a legitimate message classified as spam?

Spamassassin spam filtering assigns points to each characteristic of spam which it can identify. A significant number of points are added to spam which contains link to websites used exclusively by spammers. Additionally, a combination of badly formatted email written using non-standard clients and spam-like content may be identified as spam.

All internal Penn email are given a score of -12 by default, but email coming in from outside may be subject to false positives if your spam filtering level is set to "Very High" or "High". It is good practice to check your caught-spam folder once in a while to see if there are no false positives.