I suggest making it possible add some kind of Spam filter or email filter system. For accounts with to much spam is not possible use Roundcube. I am talking about account wiht 150 junk emails per day.
I think this is VERY needed.
It may be, but Roundcube is just a web client. You're just supposed to use Roundcube to read your emails and send them, not do everything else!
While it is nice to do the spam filtering at the MTA level, some people aren't lucky enough to have that kind of control.
This is something that a third-party could work on once a plugin system is in place, but I wouldn't say it is so important that it's required to be in the base system.
Well, i think a simply filter could be good.
Not something like a complete spam filter, but something to hunt some spam from the messege subject or something like that.