The Lenny version of the spamassassin package contains a regular expressions that considers as “coming from the future” mails dated in 2010. Starting from a couple of days ago, that has increased the possibility of false positives.
More information about that can be found in some blog posts such as this one and in the Debian BTS, bug #563245.

If you are running spamassassin on a Debian machine, you should probably upgrade it to avoid false positives. That can be easily achieved using the volatile version of spamassassin 3.2.5-2+lenny1.1~volatile1. More information about this update and instructions of how to update in this announcement from the debian-volatile-announce mailing list.

Generally, it is a good idea to have a volatile line in your sources.list. Read more about the debian-volatile project in their website.

Unstable users only need to make sure they update to the latest version available in the archive 3.2.5-7.

Contributed by: Stefano Zacchiroli