O Postfix verificará os mapas na ordem especificada até encontrar a primeira correspondência e entregar o email a esse destinatário.
Trecho de man 5 postconf
alias_maps (default: see "postconf -d" output)
The alias databases that are used for local(8) delivery. See aliases(5) for syntax details. Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will be searched in the specified order until a match is found. Note: these lookups are recursive.