Sounds like a call for a custom webmail client, instead of users with panel access logging into webmail and Plesk, they go straight to plesk and the webmail is on there. (Obviously not for domain administrators/client login details, but directly for mail account users/POP auth info)
Custom webmail client making it more Hotmail/Yahoo like would be very sexy and at that point plesk have control of the spam filtering at a very high level, enabling the spam_train service to be built in to the mail server rather than a separate process (Handy for server load). Only real complexity is would Parallels write a webmail client, as it would not be very professional to use something like Horde or RoundCube, etc....
An interesting thought, perhaps something there for plesk 12/13??
Custom webmail client making it more Hotmail/Yahoo like would be very sexy and at that point plesk have control of the spam filtering at a very high level, enabling the spam_train service to be built in to the mail server rather than a separate process (Handy for server load). Only real complexity is would Parallels write a webmail client, as it would not be very professional to use something like Horde or RoundCube, etc....
An interesting thought, perhaps something there for plesk 12/13??