I've actually read at the begining of the Virtual Users And Domains With Postfix, Courier And MySQL (+ SMTP-AUTH, Quota, SpamAssassin, ClamAV) to use The Perfect Setup - Debian Sarge - page 1 and page 2. That's why I've started to install it that way.
But... I already have a problem :-)
At the place where I have to choose http mirror for apt, after my choice is made, I got a message telling me that there's temporary failure resolving hostname of chosen mirror, and no matter what selection I make, the same message keeps appearing...
What to do now?