I have rewritten my How to set up a mail server on a GNU / Linux system , which is now based on Ubuntu Hoary.
(Used to be Mandrake, but is now completely rewritten from scratch to use apt-get, The old one was listed on postfix.org)
It is a complete step by step guide to install, configure and run these packages:
Ubuntu + Postfix + Courier IMAP + MySQL + Amavisd-new + SpamAssassin + ClamAV + SASL + TLS + SquirrelMail
Not sure whether this should be in the server forum, but it is a howto hence the tips forum.
I must be wrong, but I could not see any other mail server howtos for ubuntu mentioned in the forums.
Feel free to send me comments.
All the installation and configuration sections are complete. Some of the later testing and appendix needs padding out, but as any other document it will forever be in development.