[Exim] case and alias problem

I have a weird problem here. I've migrated our last mailserver from
sendmail to exim and ofcourse small things pop up that I fix as I go.
But this one has me stumped.

Like many IPSs we alias user@theirdomain to localuser. Now some users
have in their .forword in their homedir a copy of their own address,
to force local delivery, and another address to forward it to. Like
this:

user@???:    duser

~duser/.forward:
user@???
user@???

This actually works. However, if I send mail to User@???
instead of user (capitals somewhere in the localpart OR domain)
it doesn't work - it says "user unknown" for the local user alias.

Here's a way to reproduce it:

##== exim.conf ==##

local_domains = traveler.cistron-office.nl:book.cistron.net

# .....

# FIRST entry in the directors list
virtaliases:
driver = aliasfile
domains = book.cistron.net
file = /etc/mail/virtaliases
search_type = lsearch
include_domain
expand
no_more

##== /etc/mail/virtaliases ==##
somealias@???:    stuser

##== ~stuser/.forward ==##
somealias@???

Result:

# exim -v -bv somealias@???
stuser@???
    <-- stuser@???
    <-- somealias@???
  deliver to stuser in domain traveler.cistron-office.nl
  director = localuser, transport = local_delivery

This works, mail for somealias@??? is delivered locally to stuser.

However:

# exim -v -bv Somealias@???
somealias@??? failed to verify:
unknown local-part "somealias" in domain "book.cistron.net"

Note that in the error message, "somealias" got lowercased anyway.

Now if I replace somealias@??? in ~stuser with "stuser"
(which should be the same) it DOES work:

##== ~stuser/.forward ==##
stuser

# exim -v -bv Somealias@???
stuser@???
    <-- stuser@???
    <-- Somealias@???
  deliver to stuser in domain traveler.cistron-office.nl
  director = localuser, transport = local_delivery

Is this a bug, or is there some setting I forgot?

Oh, I tried both exim 3.32 and 3.34. Packaged by the Debian maintainer.

Mike.
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