Most often this occurs on Plesk based Linux systems.

Here we will list the most common cause where we have seen this error occur.

Causa #1

Premium antispam on Plesk uses a milter service listens on port 12768. If the milter service goes down (crashes) for any reason then the Plesk server will issue the 4.7.1 temporary error. MX Guarddog will queue your email when this error is returned to us, holding the mail for a maximum of 7 days. While the antispam feature is one service that uses the milter service there are actually several other features that rely on the milter service as well, including:

• greylisting
• SPF
• quota-check
• Kaspersky Antivirus
• Dr. Web
• Autoresponder
• SpamAssasin
• dk_check (domain keys)
• maillisting

Using any of the above features with a faulty milter service will generate this error. This problem has been plaguing Plesk installs for many versions, Plesk has documented the issue and solutions on their website:
https://support.plesk.com/hc/en-us/articles/213918165-Premium-Outgoing-Antispam-Mail-delivery-fails-451-4-7-1-Service-unavailable-try-again-later

Other Causes

A faulty milter service is the primary cause of this error, of course there may be other reasons and other email platforms where this error could occur. If you experience this error and it is not on a Plesk server please contact us so we can investigate and document the cause for others who encounter the same error.