How to hand over mails from postfix to cyrus

[Oliver Demetz - Hardware-XPress.de]

Hi!
I searched for hours now for a documentation on how postfix hands over
mail to cyrus.

An Example:
I want to host an emailadress xxx at xyz.com.
This Mail shall be stored in a mailbox called xxx-xyz-com for
example. So, how can I tell postfix and cyrus to do that???

What I already found out:

The commandline, which is executed by Postfix (defined in master.cf)
is:
pipe user=cyrus argv=/usr/cyrus/bin/deliver -e -r ${sender} -m ${extension} ${user}
so, deliver is told to store the mail into a mailbox. My question:
what's the value of $extension and $user???
Is $user always = cyrus (as defined at the beginning of the line?
Why do I even need to specify a user? Wouldn't it be enough to tell imap the
destination-mailbox?
And, in the case that user is cyrus, from where takes postfix the
password for cyrus to authenticate against cyrusimapserver???

I'm really confused.
Please help me getting light into the dark!

So long and really BIG THANKS to all of you in advance!

Oliver Demetz