sendmail, 4th Edition

The X Configuration Command

When the MILTER Build-time macro is enabled, sendmail offers a way to submit messages to external programs that can be used to screen messages for spam indicators, viruses, or other content that you might want to reject, defer, or quarantine. At the end of this chapter, we will show you the library routines to use for making these decisions. Here, we discuss the hooks inside the configuration file that allow sendmail to exchange information with external programs.

External programs are defined for use with the X configuration file command. The form for that command looks like this:

Xname, equates ...                         ← cf file
INPUT_MAIL_FILTER(`name', `equates ...')   ← mc file
MAIL_FILTER(`name', `equates ...')         ← mc file

The X in the first line, like all configuration commands, must begin a line. It is immediately followed by the name you will assign to the external Milter program, with no intervening spaces. That name is for sendmail’s use only, and does not need to be the actual name of the program. The name is followed by a comma. If you accidentally prefix the name with a space (in the cf or mc form), or omit the name, the following error will print and the sendmail program will exit:

cf file: line number name required for mail filter

The equates is a sequence of comma-separated expressions that are formed by a key letter, an equals sign, and a value:

key-letter=value

The recognized key letters and their meanings are shown in Table 26-3.

Table 26-3. X configuration command ...