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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: 550 Host unknown: incorrect DNS settings

Updated

By Duane GarnerChief Executive Officer (CEO)
Summarize: Claude ChatGPT View Markdown
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A domain in the message, usually the recipient's, has no mail records in DNS. Check the spelling of the address and the domain's MX records.

Shown by SMTP-based sending (Other SMTP, Zoho Mail, the backup SMTP transport)

The test result or the error log shows a reply such as 550 Host unknown or 550 5.1.2 Bad destination host, with the word "host unknown".

This article is for site owners sending through an SMTP server whose message is refused because a domain could not be found.

What this error means

Reply code 550 means the request was refused permanently. With "host unknown", the reason is DNS: the mail server tried to find where to deliver the message, or tried to check the sender's domain, and found no usable record. Without a mail exchanger (MX) or address record, there is nowhere to send the mail, so the server gives up.

Retrying will not help until the DNS problem or the address is fixed.

Common causes

  • A typo in the recipient's domain, such as gmial.com, example.con or a missing letter.
  • The recipient's domain has expired or its DNS has been removed.
  • The domain in your From Email has no MX or address records, so the server will not accept mail claiming to come from it.
  • A plugin sends to a placeholder address, such as admin@localhost or you@example.test, that cannot exist on the public internet.
  • A recent DNS change at the recipient's or your own domain has not spread yet.

How to fix

  1. Read the full reply

    Open the error in CRM-NG SMTP → Tools → Debug Events. Note which domain the server could not find.

  2. Check the address

    If it is a recipient, check the spelling. Correct it in the plugin or form that sent the email.

  3. Look up the domain's DNS

    Use a public DNS lookup tool to check the domain's MX record. No result means the domain cannot receive mail.

  4. Fix your own domain if needed

    If the unknown domain is the one in From Email, publish MX records and SPF at your DNS host, or change From Email on CRM-NG SMTP → Settings → General to an address on a domain that has them. Save.

  5. Send a test

    Go to CRM-NG SMTP → Tools → Email Test, fill in Send To with a known working address 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 address no longer fail.

Notes

  • WordPress sends some messages to the admin address under Settings → General in WordPress. Make sure that address is real and spelled correctly.
  • CRM-NG SMTP's domain check after a test looks up your From domain's SPF and DMARC; a "could not be determined" or "no SPF record" result is a hint that the From domain's DNS needs attention.
  • The backup SMTP transport never retries permanent recipient errors.
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