This feature is still offered, but as of V8.10 it does nothing. Instead, beginning with V8.7 sendmail, you should either use the service-switch file (ServiceSwitchFile) to control use of DNS or compile a sendmail without DNS support (NAMED_BIND).

The V8.6 sendmail configuration files usually assumed that you were using DNS to get host information. If you did not have DNS available (for example, if you were on a remote UUCP node), you could declare that fact:

FEATURE(`nodns')

This told sendmail to assume that DNS was not available. It did this by ignoring any errors from DNS lookups. Note that this was really only a minor fix because other features (such as bestmx_is_local) used DNS anyway.

Note that the result of a hostname lookup failure differs depending on whether DNS is used. If a hostname is looked up with DNS and not found, the message is bounced. If a hostname is looked up with DNS and there is some temporary failure, the message will be queued, and the name will be looked up again later. If the host is looked up in /etc/hosts and the lookup fails, the message bounces.

Note that this feature is deprecated, and might not appear in future versions of sendmail.