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✉️ Email Delivery

Documentation, reference material and tutorials for CRM-NG SMTP Email: setup guides for every mail service, troubleshooting, and one article per error message.

Error: 521 Server does not accept mail

Updated

By Duane GarnerChief Executive Officer (CEO)
Summarize: Claude ChatGPT View Markdown
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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 never retries permanent recipient errors like this one.
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