Having trouble emailing a domain. When trying to email any address on
this domain, I get the following back in my gwia logs.

15:03:12 116C MSG 4263532 Analyzing result file:
15:03:12 116C MSG 4263532 Detected error on SMTP command
15:03:12 116C MSG 4263532 Command: RCPT TO:<email_address>
15:03:12 116C MSG 4263532 Response: 421 Service not available, closing
transmission channel

If I try telnetting to either of their mail servers on port 25 from my
gwia, I get this:

ccg-gwia:~ # telnet mailserver_IP 25
Trying mailserver_IP...
Connected to mailserver_IP.
Escape character is '^]'.
220 mailserver.domain.com ESMTP Mail Gateway

Of course they say it's on our end, and we *can* email other external
domains, just not this one.

Any suggestions?