sendmail, 4th Edition

FallbackMXhost

Fallback MX host V8.4 and later

At sites with poor (connect-on-demand) or unreliable network connections, SMTP connections can often fail. In such situations, it might not be desirable for each workstation to queue the mail locally for a later attempt. Under V8 sendmail, it is possible to specify a fallback host to which the mail should instead be forwarded. One such host might be a central mail hub machine.

The FallbackMXhost option specifies the name of a mail exchanger machine (MX record) of last resort. It is given an artificially low priority (high preference number) so that sendmail tries to connect to it only if all other connection attempts for the target host have failed.

Beginning with V8.12, the host specified for this option has its MX records looked up, and those records are added (with artificially high preference numbers) in place of the host. This can be prevented (and the old behavior emulated) by surrounding the hostname with square brackets.

Note that this fallback MX host is used only for connection failures. Prior to V8.10, it is not used if the name server lookup fails. Beginning with V8.10, this fallback MX host is also used if the name server lookup fails. This option is available only for the [IPC] delivery agent (The use of $h in A=TCP on page 739). Note that MX lookups are available only if sendmail is compiled with NAMED_BIND defined (NAMED_BIND on page 124). Also note that, beginning with V8.13, a FallBackSmartHost option has been added ( ...