As far as I know, Mail-in-a-Box provides an user with a mail server, a roundcubewebmail, ownCloud - files, contacts & calendars, static sties etc.
I believe most people generally need a website, an email address, an instant messaging system, file sharing, storing and syncing system and of course, a contacts & calendars syncing system.
Wouldn’t it be a nice and generally complete system if you integrate XMPP/Jabber server into Mail-in-a-Box?
If really required, you could easily install prosody (XMPP):
sudo apt-get install prosody
Then configure for your host(s).
Then configure the external authentication plugin to authenticate with dovecot or postfix using the users email and password.
Should not take more than a few minutes of time and maybe a little research.
On another note, if you really want Open and free IM’ing right now, I host a server that can do just that: https://airchat.urgero.org The web site is for group chatting but registering and then signing in with pidgin or other XMPP client also works fine.