Serious Spam Stopping Techniques
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Spam or UCE (Unsolicited Commercial E-Mail) is becoming an ever-larger problem. At the same time, more Spam-fighting resources are becoming available. Below you will find some information on how to configure your server to block incoming spam.

Note: Any anti-spam configurations you employ may also block legitimate E-Mail from being delivered. The techniques covered in this document are some of the more effective anti-spam techniques available, but do NOT guarantee anything. Some spam is likely to get through, and some legitimate messages may get blocked.

Configuring Sendmail to Block Spam

The default sendmail configuration file on your server includes some very basic anti-spam tools. The first of which is POP before SMTP Relay Authentication or SMTP-Auth, which prevents spammers from using your server to distribute their messages. You also have an spammers list which will block E-Mails originating from addresses you add to the list.

By installing the sendmail M4 configuration macros, you are given access to a few additional tools. The access list is similar to the spammers list, but has added flexibility.

Content-based Filters

Content based filtering is one of the most effective means of blocking incoming spam because it checks incoming messages for specific spam-like content. SpamAssassin is a powerful and easy to configure mail filtering tool which scores incoming messages based on a number of tests. If you want to set up your own mail filters, you can run Procmail, a relatively easy to use mail filtering and redirection program. To get started using procmail to filter spam, see:

Advanced Mail Filtering for Virtual Servers

In addition to the suggested filters, you can have procmail filter messages based on keywords that come from the subject or body of the message. See the Procmail Man Page for help configuring your own filters.

Block Lists

Using a block list is an easy way to block a majority of spammers. The greatest advantage of a block list is that somebody else maintains the list of known offenders, reducing the effort on your end considerably. Probably one of the most respected black list providers is mail-abuse.org (MAPS). In order to use the MAPS RBL, you must first get an account with them. See the Fee Structure page to see the types and costs for MAPS RBL accounts. Once you have an account, use the sendmail M4 macros to install the default-spamfilters configuration file.

There are a large number of other Black List providers in addition to MAPS. Since each Black List provider has different policies on how they block spam and black list availability, you should check carefully to see if they work for you. The following link has a list of several Black Lists that are available for use:

• http://www.declude.com/JunkMail/Support/ip4r.htm

 

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