[Exim] preserving '+extension' case in local_part

Hi folks,

By default Exim lowercases all of an address (so Unix usernames which are
typically all lower-case match). What I would like to do is lowercase the
username of the local part as usual but *preserve* the case of the part which
follows the "+" i.e. the 'extension' part. To explain by example:

Original                           Rewritten

JBloggs+WorkFolder@???  ->  jbloggs+WorkFolder@???
+Shared.SomeFolder@???  ->  +Shared.SomeFolder@???
jbLOGGS@???             ->  jbloggs@???
:
etc.

I want this for Cyrus IMAPD which is case sensitive. I would like to use
mixed case folder names so I need some way of doing the above in a dynamic
on-the-fly manner. Any ideas?

Regards,

--
Cillian