Please take a look at our afterlogic webmail package. It has open source version (lite) and professional.
I suggest you contact cPanel and ask them to consider you.
You might also want to create a very robust (ie: NO BUGS) migration tool for roundcube, horde, squirrelmail... so that it is possible to easily convert users from one system to yours.
ie: Cpanel isn't going to replace any of the established webmail systems without SERIOUS consideration for migration of the many thousands of existing users.
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