With a level 2 or greater configuration file (see the V configuration command in Section 17.5), V8 sendmail passes the user part ($u) of local recipient addresses through the localaddr rule set 5 as a hook to select a new delivery agent. If the F=5 flag (F=5) is set for the delivery agent, the localaddr rule set 5 is called after all aliasing (including the ~/.forward file). The -d29.1 (a.k.a. -d29) debugging switch causes the address to be printed as it appears before the localaddr rule set 5 rewrite:

maplocaluser: output of printaddr( ) here (Section 16.3) 

Information about the address is printed with the printaddr( ) routine. The output of maplocaluser( ) becomes the input to recipient( ), so the result of rewriting can be seen by using the -d26.1 debugging switch (-d26.1) in combination with this one.