Unable to send emails to a particular domain

Hi,

In brief - we are having some trouble sending emails to a particular domain, and the messages remain in the queue with specific error messages.

The bounce message we get via email contains some generic info and the main message is:
#550 4.4.7 QUEUE.Expired; message expired ##

In the Exchange message queues - the messages appear with the following error message:

'421 4.4.1 Connection timed out. Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

What I have checked so far:

- Our firewall is open and not set to block SMTP traffic
- I can successfully telnet to the recipient domains mail server on port 25 and nslookup returns the normal information
- I have checked configs in Exchange 07 and don't really see any discrepancies but may have missed something of course
- We are not on any blacklists and MX records are ok - I can also resolve recipient MX record
- Have checked with the recipient end about their configuration and they say we are the only ones having an issue sending to them.
- We don't experience this issue with other recipients
- Attempted to add the recipient domain name to Accepted domains in EMC - no success

If anyone could shed some light on this that would be great - Thanks!