Thanks, all of you.
Summarizing, what is needed is an IMAP server which speaks lmtp with the MTA
(as per the cyrus link that Raja Subramanian sent) and imap with its clients
but handles the complexity involved (as pointed to by Binand) all by itself.
When I want to index mails then the itch is irritable enough to want to
scratch :-) Will keep you posted if I get to scratch it.
Thanks
--
Never be afraid to try something new. Remember, amateurs built the
ark; professionals built the Titanic!