Mailing Lists
We generally discuss SpamAssassin and its development via the following mailing lists. Where available, there is a graph which shows you how much mail you should expect to hit your inbox if you subscribe.
Announce
For announcements of new versions of SpamAssassin.
Subscription: send mail to announce-subscribe -at- spamassassin.apache.org
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.announce&plot.png
Users
A general user list. Please post your questions to this list. Developers and members of the PMC monitor this list and answer questions as time permits.
Note: before you submit a question to the mailing list, please thoroughly read the FrequentlyAskedQuestions list and other info on the FrontPage, as your question is likely already answered there. If you've looked carefully but still can't find your answer, then go ahead and post -- but be sure to note
how you're invoking SpamAssassin (options are in UsingSpamAssassin),
what version of SpamAssassin you're using, and
your platform.
Please don't ask questions about versions older than 2.64 and 3.0.0, as you don't want to use an old version for the same reason you wouldn't use an out-of-date anti-virus scanner.
Subscription: send mail to users-subscribe -at- spamassassin.apache.org
Unsubscribe: send mail to users-unsubscribe -at- spamassassin.apache.org
There is also a forum-style user interface for the list at Nabble.com, where you can post questions and get answers without subscribing by mail.
For 'NoMail' style subscriptions (for GMANE, etc.): send mail to users-allow-subscribe -at- @spamassassin.apache.org
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.general&plot.png
Dev
For those involved in the development effort to discuss their work on the project. Unless you are working on a patch to SpamAssassin, this is probably not a list you need to use. If you're not already on the general users list, you should probably go there first 
Subscription: send mail to dev-subscribe -at- spamassassin.apache.org
Unsubscribe: send mail to dev-unsubscribe -at- spamassassin.apache.org
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.devel&plot.png
Commits
For SVN and Wiki change messages. If you don't know what they are, you don't want to be on this list, and if you're not already on SpamAssassin-users, you should probably go there first
Subscription: send mail to commits-subscribe -at- spamassassin.apache.org
Unsubscribe: send mail to commits-unsubscribe -at- spamassassin.apache.org
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.cvs&plot.png
Dev-DE
A list for German-language rule development. Not very active currently.
Subscription: send mail to dev-de-subscribe -at- spamassassin.apache.org
Unsubscribe: send mail to dev-de-unsubscribe -at- spamassassin.apache.org
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.devel.german&plot.png
Dev-BR
A list for Brazilian rule development.
Security Issues
We treat security-related issues specially. If you believe you have discovered a security issue related to SpamAssassin, please send a mail to security -at- spamassassin.apache.org with full details.
Private Lists
The project management committee maintains a private list, used for a very limited subset of discussions that cannot take place in public. This is not open for public subscription, but is documented here for completeness and transparency. See ProjectRoles for more details on what this list is used for.
Search Across Lists
You can also search and browse a unified view of all SpamAssassin mailing lists.
Other Ways of Communication
IRC is also an option.