Roundcube looks more capable than squirrelmail, which as I've said lacks modern filters available in gmail and yahoo.
Google lets you host your mail with them, just change the mx records and allow a 1 line text ad. That sort of guts ISPCONFIG functionality though.
I recommend it be evaluated as an alternative to squirrelmail in the ISPCONFIG space.
Of course a procedure would need to be written to migrate the users mail from squirrelmail to roundcube "containers" <runs and hides>