one of the things I always liked about zimbra was the possibility to use not only the WebUI but also IMAP. As I yesterday stumbled upon a bug on IMAP SORT I found Bug #36367, closed with the remark „From Maria Lo 2011-09-23 14:33:49: Closing old bugs we're not planning to fix“. Same goes for Bug #2818 which is related.
The big IMAP sort bug, Bug #12491 „Provide IMAP SORT capability“ is still open, but there was no activity since 2009-04-15.
Given this it seems zimbra does not put much effort in consolidating and extending the IMAP interface, which is a pity.