[Exim] Getting exim ver 1.82 to accept mail from host with u…

I'm having problems with our mail server running exim 1.82. I can't find
the setting to allow it to accept email from other mail server that have a
"_" in the name. Here is an example error:

rejected EHLO: syntactically invalid argument(s): astro_usmd02.astrolink.com

I'm not very experienced in exim. It was installed and setup by an engineer
that left the company.

P.S. I understand that "_" in the dns name is not accepted in SMTP, but we
want to allow it anyway since we have numerous companies sending us email
that have "_" in the host names.

Any help would be greatly appreciated.

Thanks,

Joe