If, on the mail hub, you forget to declare the FEATURE(nouucp), the as-is address forwarded
to it will be recognized as special. Because the address was received from an
internal workstation, relaying is allowed. The ! address will have the hostA part
stripped and the result will be relayed to user@hostB.
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276 | Chapter 7: How to Handle Spam
Thus, it is a good idea to define nouucp on the mail hub if you define it on any of your
workstations.
7.4.10 FEATURE(accept_unresolvable_domains)
Beginning withV8.9, sendmail will refuse a mail message if the address specified as
part of the SMTP MAIL From: has a domain part that cannot be looked up. For example,
if the domain foo.bar does not exist, the following error will be logged with syslog
and the message will be rejected with the same error message:
553 5.1.8
... Domain of sender address other@foo.bar does not exist
If the domain cannot be looked up, the result is a temporary error:
451 4.1.8 ... Domain of sender address other@foo.bar does not
resolve
We recommend rejecting such addresses, but there might be circumstances in which
you cannot. If, for example, you are behind a firewall and lack access to full DNS
lookups, you might want to accept everything.
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