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: <you@nosuch.host>
501 5.1.8 <you@nosuch.host>... 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 will fail.

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 feature:

FEATURE(`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.