This is not supported yet in the official release. It looks like we missed this when implementing the IMAP component in 2005/2006. However, adding this was easy and it will be avaiable in the next release. If anyone is interested in a beta, please let us know at firstname.lastname@example.org!
Note: This covers RFC 4616, not the non-standard way for doing the same thing used by Exchange 2003 (which was already supported).
This is supported in Rebex Secure Mail since 2010-03 (version 1.0.3723.0 or later).