For a given name, which may or may not be a real user on this machine, you are allowed to enter:
It is possible to "pipe" all message to a program. The program can be a binary executable, a shell script, or any shell construction such as
This command, for one, will append all incoming mail for the alias in the file /tmp/mail.log.
This is the way to install mail server packages.
You may want to defer the administration of a mailing list to someone without privilege. You are allowed to specify a text file holding email names. This file may belong to one user. A simple text editor is needed to manage it.
Each email address may be simply a user name, another alias name (alias may be nested), or a full email address.
You can setup simple mailing lists this way.
Those features are independent. This means you can setup a filter program, a list file, and email addresses for the same alias name.