Role in Local Delivery
Another role of the sendmail program is to deliver mail messages to local users. A local user is one who has a mailbox on the local filesystem. Delivering local mail is done by appending a message to the user’s mailbox, by feeding the mail message to a program, or by appending the message to a file other than the user’s mailbox.
In general, sendmail does not put mail messages directly into files. You saw the exception in the aliases file, in which you could specifically tell sendmail to append mail to a file. This is the exception, not the rule. Usually, sendmail calls other programs to perform delivery. Those other programs are called delivery agents.[13]
In your sendmail.cf file you found two lines that defined local delivery agents, the ones that sendmail uses to deliver mail to the local filesystem:
Mlocal, P=/usr/lib/mail.local, F=lsDFMAw5:/|@qPSXfmnz9, S=EnvFromSMTP/HdrFromL, Mprog, P=/bin/sh, F=lsDFMoqeu9, S=EnvFromL/HdrFromL, R=EnvToL/HdrToL, D=$z:/,
The /usr/lib/mail.local program is used to append mail to the user’s mailbox. The /bin/sh program is used to run other programs that handle delivery.
[13] * Although for historical reasons, the sendmail developers still continue to use the term “mailers.”
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