Clearly this feature gives you more control over who can and cannot relay. It can be
of value at a site that is populated by some network printers and some Unix
machines. The file specified by RELAY_DOMAIN_FILE could be set up to allow the Unix
machines to relay, but not the printers.
7.4.7 FEATURE(relay_local_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 specified address, and the result is
stored in the $f macro (?§21.9.45 on page 824).
If you wish, you can use the value in the $f 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_local_from) which is
declared like this:
FEATURE(`relay_local_from??)
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
274 | Chapter 7: How to Handle Spam
Because this feature poses risk, the following warning will be printed each time you
build with your mc configuration file:
*** WARNING: FEATURE(`relay_local_from??) may cause your system to act as open
relay. Use SMTP AUTH or STARTTLS instead. If you cannot use those,
try FEATURE(`relay_mail_from??).
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