Hi,
I assume you are referring to RFC 4551: IMAP Extension for Conditional STORE Operation or Quick Flag Changes Resynchronization?
The next version of messaging server (2006Q4) won't support this extension as the RFC wasn't even finalised before development on this version ceased.
It is highly likely it will be supported in the next release, especially given that several contributors to the RFC were Sun developers.. but there is nothing guaranteed at this time.
Regards,
Shane.