Postfix allows you to store virtual alias maps in a text file, which tells postfix how to route virtual email addresses to real users on the system. This setting and the file location is determined in the postfix configuration file /etc/postfix/main.cf like so:
virtual_alias_maps = hash:/etc/postfix/virtual
The format of the file is with the alias on one side, and the destination on the other, for example like so:
john_smith@example.com john john-smith@example.com john fred@example.com john
This routes all email addressed to john_smith@example.com, john-smith@example.com and fred@example.com to the real user (or system alias) john. It’s possible to have a catch-all alias which will route anything addressed to @example.com to a particular user like so:
@example.com john
If you wanted everything to go to "john" except for mail to fred@ then you can do it like this:
@example.com john fred@example.com fred
Just editing the /etc/postfix/virtual file is not enough to make the changes take affect. You must run the postmap command to make postfix read the file, like so:
/usr/sbin/postmap /etc/postfix/virtual
This creates a new file called /etc/postfix/virtual.db and the aliases are now loaded into postfix.