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SMTP Bounce Code 5.1.2: Invalid Domain

SMTP bounce code 5.1.2 indicates that the domain portion of the recipient's email address is invalid or doesn't exist. This is a permanent failure (hard bounce), meaning the domain itself is invalid and should be removed from your mailing list.

What Does 5.1.2 Mean?

The enhanced status code 5.1.2 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 1 = Addressing status (related to the recipient address)
  • 2 = Bad destination system address (invalid domain)

When you receive a 5.1.2 bounce, it means the domain part of the email address (after the @) is invalid, doesn't exist, or cannot receive email. This is different from 5.4.4 (unable to route) in that the domain itself is confirmed to be invalid.

Bounce Type

  • Type: Hard bounce (permanent failure)
  • Category: Addressing
  • Action Required: Remove the email address from your mailing list immediately

Common Causes

  1. Domain Doesn't Exist: The domain name in the email address doesn't exist in DNS
  2. Typo in Domain: The domain name contains a typo (e.g., example.cmo instead of example.com)
  3. Domain Expired: The domain registration has expired and the domain is no longer active
  4. Domain Deleted: The domain has been deleted or deactivated
  5. Invalid TLD: The top-level domain (TLD) is invalid or doesn't exist
  6. Domain Format Invalid: The domain name format is invalid (e.g., contains invalid characters)
  7. Domain Not Configured for Email: The domain exists but is not configured to receive email
  8. No MX Records: The domain has no MX (Mail Exchange) records configured
  9. Domain Suspended: The domain has been suspended by the registrar
  10. Invalid Domain Syntax: The domain violates DNS naming rules

How to Resolve

For Email Marketers

  1. Remove from List: Immediately remove the email address from your active mailing list
  2. Verify Domain: Check if the domain exists by typing it in a browser or using domain lookup tools
  3. Check for Typos: Look for common typos in the domain (.cmo vs .com, .co vs .com, etc.)
  4. Contact Recipient: If you have alternative contact information, verify the correct domain
  5. Validate Domains: Use domain validation tools to verify domains before adding them to your list
  6. Clean Your List: Regularly clean your email list to remove addresses with invalid domains

For Developers

  1. Domain Validation: Implement domain validation before storing email addresses
  2. DNS Lookup: Perform DNS lookups to verify domain existence and MX records
  3. Implement Bounce Handling: Set up automated bounce processing to remove 5.1.2 bounces from your database
  4. MX Record Checking: Verify MX records exist for domains before sending emails
  5. Typo Detection: Implement basic typo detection for common domain mistakes
  6. Suppression List: Maintain a suppression list of invalid domains
  7. Log Domain Issues: Log invalid domains to identify patterns and improve validation

DNS and Domain Validation Checklist

  • Verify the domain exists in DNS
  • Check that the domain name is spelled correctly
  • Verify the domain hasn't expired
  • Ensure MX records are configured for the domain
  • Validate the domain format follows DNS naming rules
  • Check for common typos in the domain name

Related Bounce Codes

Examples

Example Bounce Message

550 5.1.2 Invalid domain
Domain does not exist.

Example Enhanced Status Code

550 5.1.2 <[email protected]>: Invalid domain - domain does not exist

Common Email Provider Responses

  • Generic: "550 5.1.2 Invalid domain"
  • Domain Not Found: "Domain does not exist"
  • No MX Records: "Domain has no mail servers configured"
  • Expired Domain: "Domain registration expired"

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses with invalid domains—they will continue to fail
  2. Validate Before Adding: Always validate domains before adding email addresses to your mailing list
  3. Use Domain Validation Tools: Use DNS lookup tools to verify domain validity
  4. Check for Typos: Implement typo detection for common domain mistakes
  5. Maintain List Hygiene: Regularly clean your email list to remove invalid domains
  6. Use Suppression Lists: Maintain suppression lists to prevent sending to invalid domains
  7. MX Record Verification: Verify MX records exist before sending to a domain
  8. Monitor Domain Status: Track domains that become invalid to identify patterns