A spam message got through! Can I send it to you so you can update the filters?
No. We really have more than enough spam already, so we don't really need you to forward spam to the SpamAssassin users mailing list, submit bugs for missed spam, or send it directly to the developers.
It's worth noting that there may already be rules to catch this spam. Try running "sa-update", see RuleUpdates. Also, RulesEmporium is a great resource for additional rules and scripts that are not included with SA by default.
Nevertheless, there are ways in which you may be able to help, if there are no rules yet.
First, if there are existing tests which nearly bring the message over the threshold, then yes, we could do with that data in our rescoring corpus of messages so that the perceptron (program used to generate the scores) will know to bring those scores a little higher. Before every major release, the SpamAssassin developers send out a call for advanced users to submit data for use in the score generation process. If you follow the SpamAssassin users mailing list (see MailingLists), maintain spam and ham corpuses, are familiar with Perl and rsync, and have CPU cycles and some bandwidth to spare, then this may be a way in which you can contribute. Also, there are similar runs nightly used for rule development which would also benefit from increased participation.
And when you do need to forward or attach spams, be sure to note that we need the original message including all the message headers; without headers, it's very likely to be ignored, as it's almost useless for filter development. Never ever cut-and-paste spams since that will almost always change the spacing and line-breaks of the message.
Also note, if you have a large collection of these, it's better to zip or tar.gz them up, instead of sending them one by one. For that matter, this method is often best for even a single message. A gzipped mbox is probably the most convenient way to ship collections of one to one hundred messages (as long as they're all examples for the same thing).
And rather than post your messages onto the list, which sends your spam to hundreds or thousands of people on the list, it's often better to post your messages on a web site, and give us a link to it. Those of us who do spam analysis can retrieve the message from you without tying up others' resources. (a common place that people on the #spamassassin irc channel post uncompressed single message examples is http://sial.org/pbot/spamassassin/)
Services like SpamCop would be more than happy to receive your spam message if your fully up-to-date SpamAssassin filters missed it. SpamCop can process the spam message and automatically send out abuse reports to the network administrators responsible for the origin of the spam. The message will also be recorded to the SpamCop database which is used by SpamAssassin to detect spam messages. Other SpamAssassin users may benefit from your efforts to report spam! If you enable the SpamCop plugin, just run spamassassin -r to report a spam message.