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
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
The forms of the FallbackMXhost option are as
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.