[exim] Underscore in domain names

Hello,

Is there an easy way (preferably without modifying source code) to
configure exim to allow an underscore in a RCPT domain name? I don't
mean only in EHLO, but really in the recipient domain name (address like
<ddts@tmn_00.domain.tld>).

I know this is not RFC2821 compliant, but I would need this
functionality in order to migrate from an old sendmail on a lan (which
accepted underscores on domain names) to a brand new exim in a DMZ. The
domain name depends heavily on a proprietary application, which is not
easily reconfigured.

Thanks