Beginning with V8.13, if
DNS lookups find no host to which to deliver, and if the FallBackSmartHost option
(?§24.9.49 on page 1031) is set, delivery is to the FallBackSmartHost defined by that option.
Beginning withV8 sendmail, $h (possibly as returned by the parse rule set 0) can be a
colon-separated list of hosts. The sendmail program attempts to connect to eachin turn,
left to right:
A=TCP hostA:hostB:hostC
Here, it tries to connect to hostA first. If that fails, it next tries hostB, and so on. As usual,
trying a host means trying its MX records first, or its A or AAAA record if there are no MX
records.
The host (as $h) is usually the only argument given to TCP. But strictly speaking, TCP can
accept two arguments, like this:
A=TCP hostlist port
The port is usually omitted and so defaults to 25. However, a port number can be included
to force sendmail to connect on a different port.
To illustrate, consider the need to force mail to a gateway machine that must always be
delivered on a particular port. First, design a new delivery agent that uses TCP for transport:
Mgateway, P=[IPC], ..., A=TCP gateway.domain $h
Here, any mail that selects the gateway delivery agent is transported over the network (the
TCP) to the machine gateway.domain. The port number is carried in $h, which usually carries
the hostname.
Next, design a rule in the parse rule set 0 that selects this delivery agent:
R$+ < @ $+ .
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