I run a bunch of cPanel servers and need a better webmail solution than what's built into cPanel. NeoMail is being phased out and Horde and SquirrelMail are just horrible.

Is there a webmail plugin or some third party software that provides a much better webmail experience and still plays nice with cPanel?

RoundCube is really nice. We use it as a webmail solution for our staff.

Many of our staff are on the go, almost continuously. Sometimes they have their laptops, sometimes they don't, so we setup RoundCube.

After they got used to SquirrelMail, RoundCube was a welcomed, and attractive change for them.

We have found Roundcube to be a good addition also although using HordeMail wasn't too bad for me. I do agree with SquirelMail though ( blah ) lol.

You may check the cPanel forum because I know many members are always asking about new email platforms.

I should also point out that RoundCube is brandable. Thus, if you want to provide a webmail solution to your customers that has your logo stamped on it....RoundCube can do this.

I wonder whether that is permitted under the GNU license...

Absolutely. RoundCube actually provide instructions on doing it. You're not allowed to remove the copyright however, but you can customize the entire interface (logo, toolbar, etc)

What are the issues with Squirrelmail? I've heard many say that RoundCube is better. I've used both, but neither struck me as being particularly good or bad. Is RoundCube POP or IMAP capable?

Yes RoundCube is both IMAP and POP3 capable.

Really? POP3 also? From their site, it says only IMAP

As far as web-based applications are concerned, you MUST use IMAP functionality to retrieve POP3.

This means that you can use purely IMAP, or IMAP to retrieve the POP3. Either way it uses IMAP functionaltiy to create the reception functionality.