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421 Service Temporarily Unavailable --- Symantec

Situation

When email is sent to a domain protected by Symantec Email  Security, our client sometimes receive a bounce message with the error "421 Service Temporarily Unavailable" as details in https://support.symantec.com/en_US/article.TECH247121.html

Common causes and solutions

The recipient domain's MX record is misconfigured, with the MX preferences inverted.

See Redirecting inbound email traffic to Email Security.cloud to learn how to set up MX records with the correct preference.

The sending IP address is being throttled by Symantec's traffic shaping service due to a high rate of perceived spam.

Once the administrator of the sending server takes action to  prevent spam from being sent through their IP address, Symantec is  willing to reset the IP reputation.

The update from Symantec Brightmail Investigation Team

Our software tags individual messages as spam. Some of our customers, at their discretion, temporarily block an IP if our software tags high percentage of mails from the IP as spam. This IP block is what you are experiencing.

Here are some suggestions to improve your reputation as a sender:

- Restrict or eliminate automatic forwarding of messages.

- Implement spam-filtering for outbound messages.

- Review your log to determine if there are any suspicious activities.

Even though our customers' IP blocks are "based" on our verdicts, we are unable to remove the block as each customer maintains its own threshold.