One of the things I forgot when listing up what a good email client should do is good integration with the filtering system. Sometimes, mail is misfiltered, so I want to rerun it through the filter. Also, when saying “move this mail”, the MUA should use the filter to decide on a sensible default. Obviously, this doesn’t work too well with IMAP, so either one has to invent an extension to the IMAP protocol or have the same filtering on both the client and the server. The latter would be an acceptable compromise to me, but both mailfilter/procmail needs to be extended (to be able to tell a program “given this mail, where would you put it, and please don’t run any side effects”) and email clients need to be extended to use it.