Exchange 2010 Queue Expired

Hi.

We have a local exchange server 2010, running on MS Small business server 2011 Standard

Problem is we can not send emails to certain domains.
It will take a couple of days before the user will get a delivery delayed and then a Queue expired message.
When I check the Queue is can see the emails sent are there always with the last error:

451 4.4.0 Primary target IP address responded with: "421 4.4.2 connection dropped due to SocketError " attempted failover to alternate host, but that did not succeed.

If I go to the messages it shows 421 Service temporarily unavailable.

If I google the first message I get information on Symantec Email Security. Problem is we do not use it and I phoned the IT departments of the affected domains we can not send to, only one of them used this service and there were no records of our email being blocked.

Any assistance would be appreciated.