Microsoft Exchange 2010
Every email we attempt to send (to many different target domains) is queued and has the 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. Either there are no alternate hosts, or delivery failed to all alternate hosts.
This suddenly happened out of the blue and everything was fine for many months previously. The server and local network has no problems connecting to the Internet for web browsing. Plus Exchange can still receive emails externally. It is just the sending of emails.
Any ideas?