Help ID Postfix Config Entry?

I have this line in my /etc/postfix/main.cf file and it is commented out however I am not really sure I understand what it does if I enable it.

#mailbox_command = procmail -a "$EXTENSION"

I Googled "procmail" and I think this is something I would like to use to filter SPAM messages from ever making it to my inbox.

Can anyone explain?

bump for help.

Once postfix determines a message is to be delivered locally, it [ by default ] hands it off to the local delivery agent.

That sould look like this:

(SMTP) or (local submission) -> (Postfix) -> (local) -> (mailbox)


You can change that behavior by setting the mailbox_command parameter in /etc/postfix/main.cf.

For instance, if you would like local messages to be handed off to procmail, instead of the default local delivery agent, you would set:
mailbox_command = procmail -a "$EXTENSION"

The it would look like this:
(SMTP) or (local submission) -> (Postfix) -> (procmail) -> (mailbox)

procmail is a nice server side way of filtering messages based on header, subjects or content into say different folders (if you use maildir) instead of needing clients to setup their own filtering rules. The normal postfix delivery agent doesnt provide the same functionality.

Unfortunately it has a ridiculously cryptic syntax. Useful to know though if you have an hour to experiment with it.