sendmail, 4th Edition

FEATURE(relay_mail_from)

During an SMTP conversation, the sending host specifies the address of the envelope sender by issuing a MAIL From: SMTP command. RFC2822 commentary and DSN extensions are then discarded from that address, and the result is stored in the $f sendmail macro ($f on page 824).

If you wish, you can use the value in the $f sendmail macro to determine whether a message should be relayed to any outside or inside host. Although such a method is fraught with risk, it is still made available with the FEATURE(relay_mail_from) which is declared like this:

FEATURE(`relay_mail_from')

Because this feature poses risk, the following warning will be printed each time you build your cf file from your mc file:

*** WARNING: FEATURE(`relay_mail_from') may cause your system to act as open
        relay.  Use SMTP AUTH or STARTTLS instead.

By declaring this feature, you cause the address in the $f sendmail macro to be prefixed with a literal From: and looked up in the access database (The access Database on page 277). If it is found in that database, and the value returned is a literal RELAY, that address is allowed to be relayed:

From:[e-mail]    RELAY       ← this sender can relay

If you want to base the decision to relay on a domain instead of on an individual’s address, you can declare this feature with an additional argument that is a literal domain:

FEATURE(`relay_mail_from', `domain')

With this extra argument in place, the domain part of the address in $f (the portion of the address to the right ...