So I tried roundcube out on a large and in use install. It was competing with SquirrelMail at the same time. But it had its own server.
The IlohaMail backend is too slow to consider for the site (700k+ emails) and made working with RC unbearable. I'm not sure I have the time to convert the backend to what SquirrelMail uses, but depending on how the various Courier-IMAP issues get worked out, I may make a second try at it.
Figured I would pass along the experience.
-- Christopher A. Watford christopher.watford@gmail.com