[Exim] Case Sensitivity

Stumped:

Running Exim 4.24-2 from FreeBSD ports..

As nearly all of our userbase utilize mixed-case UID's & email
'local_part', I am attempting to put Exim's UID/PWD checking back into
conformance with UNIX in general and the RFC in particular w/r
preserving case on the 'local_part' of the address - both for user
lookup and mailstore directory.

Exim.org online docs for 4.3 explain 'caseful_lower_part' and '+caseful'
and indicate that these should be in the routers wherein users are being
validated against <your auth method>.

I don't see the 4.2X docs online, but some searching turns up a few
references to: 'locally_caseless'.

Exim throws an error with the number of the first line of either of the
above that it encounters at startup, and dies. (very helpful error
handling, at least).

Where should I look for more information on preserving 'local_part' case
throughout if possible, and w/r UID auth and Maildir directory tree at
the very least?

TIA

Bill Hacker