Well I have my mail server up and running. I have found this Howto on setting up a mail server which can host multiple domains.
Rather than reinvent the wheel I have been following it. I have also been using a book called "The book of postfix" by Ralf Hidebrandt and Patrick Koetter, published by No Starch Press.
The simple answer "Because I can" is probably premature at this stage. The main reasons I want to do it are principle and security.
As Paul points out in his page on encrytping e-mail [[How to Encrypt emails in Debian Etch 64 bit, PCLinucOS and Ubuntu Hardy Heron.]] plain e-mail is inherently insecure. It is sent in plain text through servers you do not control including your ISP's. However even if you encrypt your e-mails, as Paul suggests, an attacker who can monitor your internet connection can learn a great deal about you by studying who you send e-mails to and who you receive e-mails from. In security jargon this is called "Traffic Analysis".
This is a project to build a mail server. I will be using these pages to document my design as I go along. Hopefully it will end up as a Howto and also perhaps a tutorial on Linux and e-mail. Although I am building this project for myself, I would like it to become a group project.
Anyone suggesting improvements or pointing out errors or omissions will be welcome. There are many resources available to help in this project, I will be using as many as I can and will be acknowledging and, hopefully, building on them.