The Mailer Definitions Section:

The Mailer Definitions section defines the instructions used by sendmail to invoke the mail delivery. The specific rewrite rules associated with each individual mailer are also defined in this section. Mailer definitions begin with the mailer command (M). Searching through the Mailer Definitions section of the Red Hat configuration file for lines that begin with M produces the mailer definitions in the listing below.

---

Mlocal,	P=/usr/bin/procmail, F=lsDFMAw5:/|@qSPfhn9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, T=DNS/RFC822/X-Unix, A=procmail -t -Y -a $h -d $u
Mprog,	P=/usr/sbin/smrsh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/, T=X-Unix/X-Unix/X-Unix, A=smrsh -c $u
Msmtp,	P=[IPC], F=mDFMuX, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP, E=\r\n, L=990, T=DNS/RFC822/SMTP, A=TCP $h
Mesmtp,	P=[IPC], F=mDFMuXa, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP, E=\r\n, L=990, T=DNS/RFC822/SMTP, A=TCP $h
Msmtp8,	P=[IPC], F=mDFMuX8, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP, E=\r\n, L=990, T=DNS/RFC822/SMTP, A=TCP $h
Mdsmtp,	P=[IPC], F=mDFMuXa%, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP, E=\r\n, L=990, T=DNS/RFC822/SMTP, A=TCP $h
Mrelay,	P=[IPC], F=mDFMuXa8, S=EnvFromSMTP/HdrFromSMTP, R=MasqSMTP, E=\r\n, L=2040, T=DNS/RFC822/SMTP, A=TCP $h
Mprocmail,	P=/usr/bin/procmail, F=DFMSPhnu9, S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP/HdrFromSMTP, T=DNS/RFC822/X-Unix, A=procmail -Y -m $h $f $u

---

The first two mailer definitions in the previous listing are required by sendmail. The first of these defines a mailer for local mail delivery. This mailer must always be called "local." The P argument defines the path to the local mailer. In this configuration, procmail is used as the local mailer. The second definition is for a mailer that delivers mail to programs, which is always called "prog". The P argument points to a program named smrsh, which is the Sendmail Restricted Shell—a special shell program specifically for handling mail. sendmail expects to find both of these mailers in the configuration, and requires that they be given the names "local" and "prog". All other mailers can be named anything the system administrator wishes. However, in practice, that is not the case. Because the sendmail.cf files on Linux systems are built from the same m4 macros, they use the same mailer names.