Dovecot is a mail server whose major goals are security and extreme reliability. It tries very hard to handle all error conditions and verify that all data is valid, making it nearly impossible to crash. It supports mbox/Maildir and its own dbox/mdbox formats, and should also be pretty fast, extensible, and portable.
See also all dovecot packages.
A good starting point is to read the documentation located in /usr/share/doc/dovecot-core.
In conjunction with Exim
Dovecot can be used in conjunction with Exim to create an SMTP-AUTH mechanism for authenticating SMTP requests.
If you're using Exim and trying to connect to Dovecot's auth-client and you receive this message; "435 Unable to authenticate at present: authentication socket connection error" You ought to check the ownership of the Dovecot authentication client socket. It runs in /var/run/dovecot/auth-client and you can see its ownership there. If you have set it to "mail" or similar you'll likely need to change it to Debian-exim so Exim can read and write to the socket.