sendmail, 4th Edition

Delivery Through a Program

Mail addresses that begin with a | character are the names of programs to run. You saw one such address in the example aliases file:

ftphelp:       |/usr/local/bin/sendhelp

Here, mail sent to the address ftphelp is transformed via an alias into the new address |/usr/local/bin/sendhelp. The | character at the start of this new address tells sendmail that this is a program to run rather than a file to append to. The intention here is that the program will receive the mail and do something useful with it.

The sendmail program doesn’t run mail delivery programs directly. Instead, it runs a shell and tells that shell to run the program. The name of the shell is listed in the configuration file in a line^[14] that begins with Mprog:

Mprog,   P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,

In this example, the shell is the /bin/sh(1). Other programs can appear in this line, such as /bin/ksh(1), the Korn Shell, or smrsh(1), the sendmail restricted shell that is supplied with the source distribution.

^{[14] *} Actually, delivery agent definitions often span multiple lines.