This is using MS SBS2011 exchange server. My user told me that he got the delayed mail delivery for mails attempted to send to few domains. The error message:
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 succeeded. Either there are no alternate host, or delivery failed to all hosted host.