7.4.8 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 (?§21.9.45 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 withrisk, it is still made available withth e 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.
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
7.4 Relaying | 275
By declaring this feature, you cause the address in the $f sendmail macro to be prefixed
witha literal From: and looked up in the access database (?§7.5 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:bob@your.domain 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 of the @ character) will be prefixed with a literal From: and
looked up in the access database.
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