3.2 With the Debian packages

• Install courier

    # apt-get install courier-base courier-pop courier-imap
  

• Install postfix, either using dpkg with the packages provided on my site (safest, after downloading them), with something like:

    # dpkg -i postfix*
  

  or using apt with:

    # apt-get install postfix.*
  

• Install courier-authpostfix.deb with:

    # dpkg -i courier-authpostfix.deb
  

• Answer the questions and you should be done.
• You may want to set pins up in /etc/apt/preferences in order to avoid having the packages overwritten on the first apt update-upgrade.