[Exim] headers_check_syntax

ARGH. Microsoft is the bane of my life.

I recently thought it would be useful to enable headers_check_syntax,
to cut down on the amount of crap mail coming in..

Unfortunately, it seems that LookOut! Express uses an invalid To:
header when a user is sending with only a Bcc address.

OE 4 uses:

To: <Undisclosed Recipients>

And it looks like with OE 5, MS thought maybe they were fixing it, but
didnt quite get it, with:

To: <Undisclosed-Recipient:;>

Having all these people call our tech department to fix this problem
would be bad enough, but I looked through OE 5's settings, and I can't
even find a way to change this to the valid:

To: : ;

So fixing it from the client seems to be out, even though that would be
the Right(TM) thing.

What I would sort of like to do, is either be able to specify a list of
additional patterns that would be considered valid and/or for this
setting to be a host-list, so that I could apply it to remotely
generated messages, but not to ones sent by our customers from our
network..

In fact, all the check_* options are good candidates to be host-lists
to apply the check to instead of booleans. One could use *, or !*, to
get the same behavior as a boolean.. There might need to be a special
way to specify wether to apply the check to locally-generated messages
(perhaps a local_check_*, which could even be a list of UID's/usernames
to which to apply the check to, again using * to say 'all users' and !*
for none)