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 (See this section). Note that MX lookups are available only if sendmail is compiled with NAMED_BIND defined (NAMED_BIND).

The forms of the FallbackMXhost option are as follows:

O FallbackMXhost=host           configuration file (V8.7 and later) 
-OFallbackMXhost=host           command line (V8.7 and later) 
define(`confFALLBACK_MX',`host')    mc configuration (V8.7 and later) 
OVhost                          configuration file (V8.6 deprecated) 
-oVhost                         command line (V8.6 deprecated) 

Here, host is of type string and is the fully qualified domain name of the fallback host. If host or the entire option is missing, no fallback MX record is used. The effect of this option can be seen by using the /mx rule-testing command (Section 8.5.2).

The FallbackMXhost option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.