Damn, I meant to reply to this ages ago. Yes, I do understand your concerns about Gmail/Yahoo etc. I was rolling out my standard advice to people who have their mail hosted with their ISP (leave it on their servers), rather than people who maintain their own mail. Silly me.

Re the Roundcube, meh, bulk mail handling is one area that it's severely lacking on. You've probably already made your decision, but I think SquirrelMail would be best for your needs.