We are running GW8 and GWAVA

I am receiving the following in a bounce message:

Transcript of session follows:
  Command:  Data...
  Response: 420 TCP write error
This occurs whenever we send an email to any recipient at a specific domain. I can telnet to their MX and enter a message manually without a problem.

Their MX records are as follows:
example.com.           3528    IN      MX      20 mail.example.com.
example.com.           3528    IN      MX      10 example.com.
I notice that the secondary is listed first. I also notice that their secondary does not accept connections on port 25. Neither SHOULD matter, since we are apparently making the connection and getting most of the way through the SMTP conversation before the error.

They tell me that we are the only ones who are having this trouble sending mail to them.

How can I troubleshoot this from our end? I have a message in my outbox to them with status 'Transfer Delayed'. I have tried to find the GWIA log entries where delivery was attempted, but have had no luck.