Make domain directory and user directory:
mkdir hdpf.dk chown postfix:postfix hdpf.dk mkdir karsten maildirmake Maildir maildirmake -f Drafts karsten/Maildir/ maildirmake -f Trash karsten/Maildir/ maildirmake -f Sent karsten/Maildir/ chmod -R 775 hdpf.dk
That was it. Now you should have an nice mail server using Courier imap and MySQL. If you find this howto useful please link to it form your site.
Recommendated readings:
http://www.high5.net/
http://kirb.insanegenius.net/