# Error: 521 Server does not accept mail

Source: https://catalystgroup.gg/docs/email-delivery/error-521-server-does-not-accept-mail/
Updated: 2026-10-19 · By Duane Garner

The server you reached does not take email at all, either for the recipient's domain or for sending. Check the address and your SMTP host.

> **Shown by SMTP-based sending (Other SMTP, Zoho Mail, the backup SMTP transport):** The test result or the error log shows a reply starting with `521`, for example `521 Server does not accept mail` or `521 5.3.2 Host does not accept mail`.

 

This article is for site owners sending through an SMTP server who get a 521 reply.

 

## What this error means

 

Reply code 521 means the server you are talking to does not accept email, full stop. It is a permanent answer, so retrying will not help. There are two places it can come from:

 
- **Your own SMTP host** refused you because the machine is not a mail server for sending, or does not accept connections like yours.
- **The recipient's side** reported that their domain does not receive email. Some domains publish a "null MX" record to say exactly this, and a relaying server passes the 521 back to you.

 

## Common causes

 
- **SMTP Host** points to the wrong machine, such as your website server, a web proxy or an old mail server that has been retired.
- The recipient's domain has been set up to receive no email, for example a domain used only for a website, or a "no-reply" domain.
- A typo in the recipient's domain that happens to land on a domain that refuses mail.
- The mail server has been shut down or replaced but its name still points to an old machine.

 

## How to fix

 
1. Read the full reply
  Open the error in **CRM-NG SMTP → Tools → Debug Events** or with **View Error Log** in the failed-send banner. Note whether it fails on every recipient or only one.
2. If every message fails: check your SMTP host
  Look up the correct outgoing server name in your mail provider's documentation. Go to **CRM-NG SMTP → Settings → General** and correct **SMTP Host**, **SMTP Port** and **Encryption**. Save.
3. If one recipient fails: check the address
  Look for a typo in the domain. Use a public DNS tool to look up the domain's MX record; a single record pointing to `.` means the domain accepts no mail.
4. Use a working address
  If the recipient's domain refuses mail, ask them for another address. If it is your own domain, publish real MX records at your DNS host.
5. Send a test
  Go to **CRM-NG SMTP → Tools → Email Test**, fill in **Send To** with an address you know works and press **Send Email**.

 

## Verify it worked

 

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." Messages to the corrected recipient no longer fail.

 

## Notes

 
- If plugins on your site send to a "no-reply" address on a domain that receives no mail, every one of those messages will fail with 521. Change the recipient in that plugin.
- The [backup SMTP transport](https://catalystgroup.gg/docs/email-delivery/backup-smtp-transport/) never retries permanent recipient errors like this one.

 

## Related

 
- [Other SMTP mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-other-smtp/)
- [Zoho Mail mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-zoho/)
- [DNS records for email](https://catalystgroup.gg/docs/email-delivery/dns-records/)
- [Error: 550 Host unknown: incorrect DNS settings](https://catalystgroup.gg/docs/email-delivery/error-550-host-unknown/)
- [Error: 550 Requested action not taken: address does not exist or was rejected](https://catalystgroup.gg/docs/email-delivery/error-550-recipient-rejected/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)