I believe the best way to do this is to open Terminal.app and make modifications to the /etc/postfix/virtual_users file manually. The postfix configuration is pre-configured to read a hash db built from this file.
The file is self-documenting, but, examples could look like:
That would add fubar and wibble as aliases to "boffo" -- this also assumes you've already configured the Hosting portion of your mail server via the GUI to include support for domain.com as a virtual domain that you host.
Once you've added the entries, you can type:
sudo postmap hash:/etc/postfix/virtual_users
And the changes should be live.
Postfix itself can be configured to work with alias files, which is pretty standard, however, this recipe is what I followed to keep my configuration as Apple-friendly as possible.
Thanks Scott! I followed the steps you suggested. First: the file did not exist, so I created the virual_users with vi an edited as advised. After send the postmap command a virtual_users.db was created. Unfortunately I still don't get mails to alias adresses. Basic username works fine alias not. Any idea?
You can use workgroup manager if you want to create multiple email addresses for a user in the same domain. for example, if you create a user named fred, his email would be email@example.com. If you add more usernames in workgroup manager, in the fred account (shortnames) like joe tom mary, then any email to firstname.lastname@example.org or email@example.com, etc will go to firstname.lastname@example.org.