[Exim] Is there a switch for these broken mailers?

I've been getting a lot of this type of message in my reject log:

rejected EHLO from [207.13.203.195]: syntactically invalid argument(s):
mkt_mail1.harborfreight.com

I know it's because of the underscore in the host name, and I know that it
is in fact syntactically invalid, but management wants to get mail from some
of these (not this particular one) malformed hostnames.

So the question is - is there a switch to allow Exim to accept mail from
these?

Elliot