Messages are accumulating in the Symantec Messaging Gateway (SMG) delivery queue with the code "451 470 Temporary server error".
This issue appears to be limited to some versions of Microsoft Exchange.
This message is returned by the destination mail server when SMG attempts to deliver the message. This does not indicate that there is a problem with SMG operation, but that the destination mail host is temporarily unable to accept mail delivery.
If messages are being delivered to the internal Exchange Server, please review the Receive Connectors configured in Exchange to ensure that they do not share the same IP address and port, and that DNS is properly configured on the destination Exchange server.