Exchange 2010 – can’t send to a specific domain – 421 connection dropped due to socket error

I've looked at a few of the existing SF questions/answers and so far can't find the same situation. It's not a TLS issue like I first thought, since the remote server isn't asking for TLS, and it isn't a firewall issue since the connection is established and starts sending and receiving SMTP commands.

We are getting the following in our Exchange queue for just one domain:

The remote domain is an Exchange 2003 server it appears.

Here's a copy of the SMTP log for the email:

2013-04-12T19:03:20.684Z,Internet,08D004A55169DB35,0,,XX.XX.XX.XX:25,*,,attempting to connect
2013-04-12T19:03:20.731Z,Internet,08D004A55169DB35,1,10.54.2.15:25977,XX.XX.XX.XX:25,+,,
2013-04-12T19:03:20.777Z,Internet,08D004A55169DB35,2,10.54.2.15:25977,XX.XX.XX.XX:25,<,"220 plexch2k3.REMOTE.com Microsoft ESMTP MAIL Service, Version: 6.0.3790.4675 ready at  Fri, 12 Apr 2013 15:03:20 -0400 ",
2013-04-12T19:03:20.777Z,Internet,08D004A55169DB35,3,10.54.2.15:25977,XX.XX.XX.XX:25,>,EHLO exchange.OURDOMAIN.com,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,4,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-plexch2k3.REMOTE.com Hello [6X.XX.X.70],
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,5,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-TURN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,6,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-SIZE,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,7,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-ETRN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,8,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-PIPELINING,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,9,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-DSN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,10,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-ENHANCEDSTATUSCODES,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,11,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-8bitmime,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,12,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-BINARYMIME,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,13,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-CHUNKING,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,14,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-VRFY,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,15,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-X-EXPS GSSAPI NTLM LOGIN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,16,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-X-EXPS=LOGIN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,17,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-AUTH GSSAPI NTLM LOGIN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,18,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-AUTH=LOGIN,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,19,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-X-LINK2STATE,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,20,10.54.2.15:25977,XX.XX.XX.XX:25,<,250-XEXCH50,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,21,10.54.2.15:25977,XX.XX.XX.XX:25,<,250 OK,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,22,10.54.2.15:25977,XX.XX.XX.XX:25,*,5945,sending message
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,23,10.54.2.15:25977,XX.XX.XX.XX:25,>,MAIL FROM:<Bob@OURDOMAIN.com> SIZE=13577,
2013-04-12T19:03:20.824Z,Internet,08D004A55169DB35,24,10.54.2.15:25977,XX.XX.XX.XX:25,>,RCPT TO:<Dave@REMOTE.com>,
2013-04-12T19:03:20.871Z,Internet,08D004A55169DB35,25,10.54.2.15:25977,XX.XX.XX.XX:25,<,250 2.1.0 Bob@OURDOMAIN.com....Sender OK,
2013-04-12T19:03:20.949Z,Internet,08D004A55169DB35,26,10.54.2.15:25977,XX.XX.XX.XX:25,-,,Remote

I can't seem to figure out what the problem is. It appears that it might be on the remote end as after the:

250 2.1.0 Bob@OURDOMAIN.com....Sender OK,

It then says:

,,Remote

whatever that means, and then that's the end of that log and connection.

Any ideas on how to resolve this one?