There are many PHP email clients, but this module might be what you are looking for:
extensions.joomla.org/component/option,c...k_id,1499/Itemid,35/
I am in the process of working on an email component for Joomla! that will provide cpanel integration such that Joomla! users can be automaticaly (with or without admin oversight) provisioned as cpanel email users.
I am also evaluating web based email clients with the idea of creating a Joomla! component out of one of them so that there is a fully integrated webmail solution there as well (think someone logs in to the site, and immediately sees an email status in one of the module locations, and, when they click on it, go to the full email interface, all within Joomla!).
Don't hold your breath for my solution, though, as there are some security issues that I am working through, and I have to work on it in between paying gigs!