A Kolab user should be able to mark an email as SPAM. This email should then be moved to a dedicated SPAM folder. Furthermore, Kolab should learn from that SPAM mail and increase its chances to classify similar mails as SPAM in the future.
Bonus points for default sieve filters that move incoming SPAM mails automatically to the junk folder, so users do not have to set those filters up manually. The SPAM filter rule could even get its own more user friendly interface similar to managesieve_vacation that lets users set the SPAM threshold and the folder to move mails to.