Prior to V8.7 sendmail, only the local delivery agent could cause addresses to be looked up in the aliases(5) database (Section 12.1.2). Beginning with V8.7 sendmail, any delivery agent that has an F=A delivery agent flag set will cause its $: address to be looked up on the lefthand side of the aliases(5) file.

For example, the F=A delivery agent flag can be used to design a local-clone delivery agent that recognizes certain nonlocal addresses as local for aliasing purposes:^[26]

^[26] This example is somewhat contrived because the same thing can be done in a more versatile manner with the mailertable feature (FEATURE(mailertable)).

R$+ <@ FIRE.WALL>     $#firelocal $: $1@fire.wall

This allows an alias file such as the following to legally exist:

George.Washington@fire.wall:   gw@internal.net

For example, as a safety net (and if the F=5 delivery agent flag is also specified), any address that is not found in the aliases(5) database will be passed to the localaddr rule set 5 (Section 19.6), where another delivery agent can be selected.

In configuration files prior to Version 6 (Section 17.5), the F=A delivery agent flag is automatically set for the local delivery agent.