[exim] exim is rejecting messages for specific domain

hello:

Checking the exim's log I have the following entries

/var/log/exim4/mainlog.1:2005-11-22 05:44:01 rejected EHLO from 
arrow-srv.arrow1                                             897.com 
[209.121.252.66]: syntactically invalid argument(s): arrow_srv.arrow1897     
                                         .com
/var/log/exim4/mainlog.1:2005-11-22 06:15:05 rejected HELO from 
arrow-srv.arrow1                                             897.com 
[209.121.252.66]: syntactically invalid argument(s): arrow_srv.arrow1897     
                                         .com

and the report that somebody from this domain cannot send emails to us.

Investigating about that I found on Internet that the charecter "_" used on
the hostname could be the reason why the messages are being rejected.

I tried to find if I can change something on the exim's configuration to
avoid this problem or take other decision without a result yet.

I'm looking for a general solution for this case.

Thanks in advance for any help
Enediel

Linux user 398956
Debian GNU/Linux