[Exim] headers_check_syntax, headers_checks_fail, exim V3

As we learn from previous discussions here, headers_check_syntax is an
all-or-nothing check, there aren't facilities for excusing some
recipients &/or senders from the checks.

We know that enabling the headers syntax check in the dept. mailer
saves us from a noticeable amount of spam, but occasionally our users
are sent mails which they would like to receive, from mailing lists
that cannot be persuaded to use headers that are acceptable to exim
v3. The issue I'm dealing with right now is a header that says

To: <Undisclosed-Recipient:;>

, about the legality of which I know different views have been
expressed.

OK, looking at the headers checks again, I found headers_checks_fail,
which it says if set to false the tests will be done, but only logged -
not used to reject mail.

My question would be then whether this checking and logging would
leave some trace which the system_filter could sense, and apply some
more-elaborate tests before deciding whether to pass the mail on, or
to bounce it.

The documentation doesn't mention whether it cuts an extra header
(along the lines of the x-rbl-warning that's injected when an RBL is
used in /warn mode), or leaves any other kind of indicator around.
It would be helpful to know this, please, or any other suggestions.

(If the suggestion is "upgrade to V4" then this _is_ intended at
some point in time, yes, but some kind of workaround would still be
useful for now.)

best regards