My user cannot send out some domains

My users cannot send email to some domains.

When I check Mail Q on the exchange server (2010), I saw the following messages for the problematic domains:

451 4.4.0 Primary target IP address responded with: "421 4.4.2 Connection dropped due to SocketError." Attempted failover to althernate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

Can you please answer the following questions.

1. what is the main reason about the problem?

2. What is the best way to solve the problem?

Thanks in advance.