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:
501 5.1.8 ... Sender domain must exist
This is the Title of the Book, eMatter Edition
Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
17.8 Configuration File Feature Reference | 615
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 FEATURE(accept_unresolvable_
domains):
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.
17.8.3 FEATURE(access_db)
A database for mail policy V8.9 and later
Prior to V8.9, the only way to accept or reject mail from selected sites was to use tcpwrappers,
or to write your own custom rule sets and rules.
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