The MAIL FROM: command of the SMTP
transaction is used to convey the address of the envelope sender.
RFC821 requires that the envelope sender address always be fully
qualified. That is, it must always have a user part, an
@ character, and a domain part, in that order.
Ordinarily, sendmail looks up the domain part of
the address using DNS, and, if not found, rejects that SMTP
transaction. For example:
MAIL FROM: <email@example.com>
501 5.1.8 <firstname.lastname@example.org>... Sender domain must exist
This is useful in blocking spam and fraudulent mail. However, if your
machine is behind a firewall, it is possible that it cannot look up
any outside addresses. In that situation, all mail from the outside
If you need to allow all mail to be received when the domain part of
the envelope sender address cannot be looked up, you can do so by
declaring this accept_unresolvable_domains
You can also declare this feature on a machine that is dedicated to a
special purpose. A machine dedicated to receiving and processing
survey reply mail might be a good candidate for this feature. If you
don't care about the spam protection offered without
this feature, go ahead and declare it.