Recipe 6.13 Bypassing Spam Checks

Problem

Your sendmail is configured to block incoming junk mail, and you have been asked to allow junk mail through when it is addressed to specific recipients.

Solution

Add an entry to the /etc/mail/access text file for each recipient who should be allowed to receive junk mail. Use the tag Spam : and the recipient address to create the key field of the entry. Use the keyword FRIEND as the return value for each entry. Run makemap to build a hash type database from the text file.

Create a sendmail configuration containing the access_db feature and the delay_checks feature with the optional friend argument. Make sure that the access_db FEATURE macro precedes the delay_checks FEATURE macro in the configuration. Here are the lines that would be added to the sendmail configuration:

 dnl Use the access database FEATURE(`access_db') dnl Check for spam friends before rejecting the mail FEATURE(`delay_checks', `friend') 

Rebuild the sendmail.cf file, copy the new sendmail.cf file to /etc/mail , and restart sendmail, as described in Recipe 1.8.

Discussion

Someone on your system ”the postmaster , a security expert, a developer writing mail filters ”might need to receive junk mail that is normally blocked by sendmail. The delay_checks feature allows this by changing the order in which spam checks are applied. The delay_checks feature allows the envelope recipient address to be checked before the envelope sender address or the connection address, which, in turn , makes it possible for mail addressed to specific recipients to bypass the other two checks. To use the delay_checks feature in this way, it must be invoked with the friend argument, as shown in the Solution section.

The specific recipients allowed to receive junk mail are defined in the access database using the Spam : tag and the FRIEND return value. Here is an example access database:

 Connect:example.com                  REJECT Spam:[e-mail]            FRIEND Spam:[e-mail]     FRIEND 

Given the sendmail configuration described in the Solution section, this access database rejects mail from example.com unless it is addressed to [e-mail] or to [e-mail] .

See Also

Recipe 6.14 provides a related example. Chapter 3 and the Introduction to this chapter provide more information about the access database. The sendmail book covers the access database in Section 7.5 and the delay_checks feature in Section 7.5.6. The Delay all checks section of the cf/README file also covers this topic.