Prior to V8.7 sendmail, only the local delivery agent could cause the localaddr rule set 5 to be called (Section 19.6). The localaddr rule set 5 is called after aliasing and before forwarding, and can be used to select a new delivery agent. Beginning with V8.7, any delivery agent with the F=A delivery agent flag set (F=A) can cause an address to be looked up in the aliases(5) file. Therefore, any delivery agent that has the F=A and F=5 delivery agent flag set will cause the localaddr rule set 5 to be called as though the agent were the local delivery agent.

In configuration files prior to Version 6 (Section 17.5), this delivery agent flag is automatically set for the local delivery agent. Note that addresses that (perhaps artificially) begin with an @ character cause the localaddr rule set 5 to be skipped (Section 19.6).