Emails in the Messaging Gateway (SMG) delivery queue or Message Audit Logs indicate that it took multiple attempts to deliver the email message with a status message similar to the following:
Delivery is periodically retried until delivery is eventually successful or the message is bounced.
The destination mail server is accepting connections but is unable to accept delivery of email messages.
This is expected behavior when the destination mail server is unable to accept mail delivery and does not represent an issue with the operation of SMG.
Usually this is a temporary situation and the administators of the remote mail system will correct the mail server issues on their end.