[KLUG Members] Issues send/receiving email to one domain

[members@kalamazoolinux.org]

I have box that I can not send or recieve emails from one particular company 
(domain).  Obviously both sides are having the same problem and both have no 
problems sending or receiving emails from anywhere else.

Here is a snip from the sendmail log:
Jun  1 04:06:05 linuxserver sendmail[2996]: h4V0mVrg001458: 
to=<user@domain.com>, ctladdr=<admin@name.domain.com> (5 00/500), 
delay=1+07:17:34, xdelay=00:00:12, mailer=esmtp, pri=3000364, 
relay=mailserver.domain1.com. [12.34.56.78],  dsn=4.0.0, stat=Deferred: 421 
SMTP service not available, closing transmission channel

Is there any way to find out which end could causing the problems.  I did 
telnet in to the othe mailserver on port 25 and type HELO and I get a "421 SMTP 
service not available, closing transmission channel"  But i am not usre if that 
is a good test as if there mail server is set-up correctly to dissallow 
relaying wouldn't I get theis error?

Thank you