[Exim] Header syntax - mailer persists retrying mail to us a…

Greetings,

We currently have the following embarrassing scenario. Please excuse
me if I don't actually name the site that is making these attempts,
but the reason should become obvious.

As a matter of policy, we have set the option to check for valid mail
headers.

headers_check_syntax = true

A sender is currently trying to send us a mail with at least a hundred
or so addressees in the To: header (exim stops logging them after
that), at least one of which is syntactically invalid. So we reject
the mail on the syntax error, and I'm taking it for granted that exim
does this correctly with a permanent error code. (If the addressee
list had been much longer, I suspect we'd have rejected it on the
"ridiculously long header" test, but that's a different matter.)

However, the remote mailer soon comes back and tries again (we know
it's the same mail being retried, because the message-id matches).
At first it was doing this every few minutes; now, at least, it has
fallen back to doing it every hour or so, but it's still leaving
swathes of junk in the rejection log due to logging these hundred-odd
addressees that are in the To: header, every time the rejection is
made.

I could easily configure our exim to reject the mail out of hand, and
then the sender would get an administrative rejection report; but the
mail is from one of our funding bodies, and I would not like to upset
them, and embarrass my colleague - the intended recipient.

But as far as I understand it, the syntax checking is an all- or-
nothing option? Or is there some way I can configure a site (IP) or
sender to be excused from these header syntax checks, which otherwise
(as we know from experience) are significantly useful in keeping out
spam?

Or would I need to turn off the syntax check glboally, wait till we've
accepted the mail, and then turn it on again?

I don't know what their MTA is, but if I try to talk SMTP to the
IP address from which this request is coming, then the response
looks (modulo linewraps and anonymizing the site name) like this:

220 cen.sor.ed mailer 3.0 (Solaris) of 15:09:02 Jun 23 2000.
Here Pleased to meet you (IDENT) (ESMTP) (contact: postmaster@???)

I don't know whether that helps in recognising it, and knowing whether
it is known to misbehave in this respect.

cheers