SMTP Error 421 indicates that the receiving mail server is temporarily unavailable and unable to accept email connections at that moment. This typically happens when a server is overloaded, undergoing maintenance, or intentionally limiting incoming connections to protect itself from abuse or spam.

Because this error is temporary by nature, messages are usually retried automatically and delivered once the receiving server is available again. This guide explains how to identify the cause of SMTP Error 421 and what steps to take when it affects email delivery from a Domains.co.za email account.

How to Resolve SMTP Error 421

1. Open the bounce-back or error notification email.

2. Confirm that the error message includes SMTP Error 421 or Temporary server unavailable.

3. Wait at least 15 to 30 minutes before resending the email.

4. Check whether the recipient’s mail server is experiencing downtime or scheduled maintenance.

5. Reduce the number of emails being sent simultaneously, if applicable.

6. Login to your Domains.co.za account.

7. From your dashboard, open the Manage Services menu and select the hosting service linked to the domain you want to manage.

8. Click Manage next to the relevant domain, then select Login to cPanel. Navigate to Track Delivery.

9. Review delivery logs to confirm the message was deferred rather than rejected.

10. Retry sending the email after the waiting period.

Additional Information

• SMTP Error 421 usually indicates a short-term availability issue on the receiving server rather than an issue with the sender’s configuration.
• Many mail servers enforce connection limits to prevent spam and may temporarily refuse additional connections.
• High-volume email sending can increase the likelihood of triggering this error.
• The error does not typically indicate blacklisting or spam filtering.
• Messages affected by this error are often delivered successfully after automatic retries.
• Repeated occurrences may require adjusting the sending frequency or batch size.
• If the error persists for several hours, the issue may need investigation by the recipient’s mail provider.

Login to Domains.co.za Account

1. Go to the Domains.co.za website Account Login page.

2. Enter your Email and Password and click the Sign In button.

3. You will see the Domains.co.za Dashboard, displaying the Manage Account menu on the left and your Account Information, Account Overview and Open Support Tickets on the right.