Mailing List Spam Filtering

Filtering senders

There are a few things you can do to address the spam problem on CSAIL mailing lists. Lists can be configured such that only list members are permitted to post. That way, the lists are still open to everybody, and spam won't get through (unless the spammer happens to use a forged return address that matches a subscriber's address). To restrict posting to list members only, click "Privacy Options" on the admin screen. Several new links will appear below it. "Sender filters" is one of them. Click that link. In the resulting page, find 'Action to take for postings from non-members for which no explicit action is defined.' and set that to Hold.

You can also define some sender filters in the 'privacy options' configuration. In the 'List of non-member addresses whose postings should be automatically accepted.', add something like

^.*@mit\.edu
^.*@.*\.mit\.edu

and set the 'Action to take for postings from non-members for which no explicit action is defined.' to either "hold" or "discard". This should open the list to all MIT affiliates (assuming they use their MIT email address in the "From" field of mail they send!) while filtering messages from outside.

Filtering based on "spam score"

There are some other bits you can set to combat the spam that involve content filtering, rather than sender filtering:

• Check the 'spam filters' section of 'privacy options'.
• In the first box there, entitled 'Filter rules to match against the headers of a message.', add X-Spam-Flag: YES and set the action to discard.

Mail to lists is examined by spamassassin. By default, mail is completely rejected if it gets a spam score of 15 or greater; it never even reaches the list management software. But if it gets a score between 5 and 15, it merely gets the X-Spam-Flag header attached. You can filter your list based on that header if you choose.

We strongly recommend against setting the action to anything other than discard when configuring spam filtering on a mailing list. Other options will cause Mailman to send email to the sender of the held or rejected message. Since 99.9% of spam is sent with a forged return address, nearly all of Mailman's messages will either bounce or reach some innocent bystander who had nothing to do with the original message.

-- NoahMeyerhans - 18 Feb 2005