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.
For announcements of new versions of SpamAssassin.
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.announce&plot.png
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. You must be subscribed to the list with the email address you use to post to it.
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
Please don't ask questions about versions older than 3.3.0, as you don't want to use an old version for the same reason you wouldn't use an out-of-date anti-virus scanner.
This list has high enough volume that you will want to filter it to a separate folder so it doesn't fill up your inbox. This is a basic function of email clients, check your mail client's documentation for instructions. If possible, it is best to filter on the header List-Id: <users.spamassassin.apache.org>
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.general&plot.png
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 You must be subscribed to the list with the email address you use to post to it.
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.devel&plot.png
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
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.cvs&plot.png
A list for German-language rule development. Not very active currently.
http://gmane.org/plot-rate.php/plot.png?group=gmane.mail.spam.spamassassin.devel.german&plot.png
A list for Brazilian rule development.
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.
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.
You can also search and browse a unified view of all SpamAssassin mailing lists.
IRC is also an option.