There are tons of great documentation out there showing you how to set this SMTP server up. Postfix is very popular and a great alternative to the even more popular sendmail. Here are some good resources that i used to learn how to set this up:
Ubuntu postifx documenation
Postfix virtual mailbox setup
Centos postfix setup
Centos postfix restrictions
Setting up dovecot is simple enough. Check out the following resources:
Ubuntu dovecot configuration
wiki.Dovecot.org virtual users
wiki.Dovecot.org virtual users example
wiki.dovecot.org authentication/password schemes
Integrating dovecot SASL with postfix
wiki.dovecot.org cram-md5 howto
Postifx/dovecot mail system can make use of actual system users or virtual users. With system users, you would have to create a new system users (eg, adduser mark) for each user. I don't really want to have to create a new system user everytime to add a new mail user. Using virtual users with virtual mailboxes suites my installations better (more personal preference). The key thing to remember is that you would have to make changes to both postfix and dovecot configutations to get this to work. In postfix, the key settings that need to be modified can be seen here. In dovecot, this example will show you how to setup the virtual user accounts for SASL login authentication.
