If you run a public mail server, there is a good chance you run SpamAssassin.
There is a New Year's SNAFU in which any dates 2010+ are marked "grossly in the future". That is a problem since it is now 2010.
Run sa-update and restart SpamAssassin ASAP, or install the following rule to local.cf:
score FH_DATE_PAST_20XX 0.0
Thanks to LWN for pointing this out to me before too much damage was done.