X-Mail is at:
http://www.xmailserver.org/It's BAY FAR the easiest unix mail system to setup. It is a single install for MTA/SMTP/POP3 out of the box. You compile/install binary, edit a few simple lints in the config, and start it....and it all works properly. It's also far faster than other solutions I have used via any other provider.
On their home page and forums are clear instructions on how to integrate with dovecot for IMAP support, as well as a host of scripts for spam/av filtering. The whole system is configured by a simple set of config files for mail domains and users/aliases. There are also several good Win32/X/PHP front ends that make it even more simple to administer.
The config files are plain english and easily configurable/readable by human beings....no macro languages in between. They are all simply:
"setting" <tab> "value" <tab> "value" <CR/LF>
There hs also a simple command line program to interface with the mailer to maintain the config files through simple commands. All that has to be done manually to get things up and running is the edit of the master config, use a utility to crypt a password for access via a control user, symlink their sendmail replacement, and start the daemon.
It would be far easier to integrate and maintain via a system like Virtualmin, and could be setup on any linux system regaurdless of the distribution very easily. Rather than having to create a complex setup for each user of directories, multiple lines in multiple config files, all that would have to be done is aither have the perl script use the command line client through an authorized admin user to add the domain and subsequent users, or have it add a single line to a single config to create the domain and the same for every user there after.
Why things have to be so complicated with sendmail/postfix/exim/qmail/cyrus/courier/sasl/dovcecot/mbox/Maildir/etc....I will never understand.