Roundcube as GUI for Postfix/Dovecot mail servers
Roundcube is an open source webmail software.
It is used as a web interface for an imap mailserver. Users can check their inbox and send email et cetera. It looks nice, a small problem is the delay until click or drag commands are executed. This is probably because a connection to the imap server is opened and the command executed, what takes some milliseconds.
The interface is responsive, that means it looks good on a mobile phone, too.
Plugins for different additional functions are available (external mailbox integration, sieve filters, encryption, ...).
There are some useful tutorials how to setup your server in the internet. Just search for it.