That program is usually /usr/lib/mail.local (or witholder systems,
/bin/mail) but can easily be a program such as deliver(1) or procmail(1).
Under Unix, a user??™s mailbox is a single file that contains a series of mail messages.
The usual Unix convention (but not the only possibility) is that each message in a
mailbox begins with a line that starts with the five characters ???From ??? (the fifth is a
blank space) and ends with a blank line.
The sendmail program neither knows nor cares what a user??™s mailbox looks like. All
it cares about is the name of the program that it must run to add mail messages to
that mailbox. In the example, that program is /usr/lib/mail.local. Th eM configuration
lines that define delivery agents are covered in detail in Chapter 20 on page 711.
1.6.6 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. Th e| 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.
* Although for historical reasons, the sendmail developers still continue to use the term ???mailers.
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