OfflineIMAP is a tool to simplify your e-mail reading. With OfflineIMAP, you can read the same mailbox from multiple computers. You get a current copy of your messages on each computer, and changes you make one place will be visible on all other systems. For instance, you can delete a message on your home computer, and it will appear deleted on your work computer as well. OfflineIMAP is also useful if you want to use a mail reader that does not have IMAP support, has poor IMAP support, or does not provide disconnected operation.
Replicate IMAP folder hierarchy to your local drive
Kolab ist eine freie Groupware-Lösung. Sie besteht aus dem Kolab-Server und den dazugehörigen Schnittstellenimplementationen in einer ganzen Reihe verfügbarer Kolab-Clients.
I have found that this is the easiest combination that allows you to serve Virtual Domains, and Users. With this it's also very easy to implement webmail systems like SquirrelMail.