Your sender domain does not exist or cannot be resolved. The receiving server looked up the domain in your From address and could not find it in DNS, meaning bounce messages cannot be returned to you.
What Does Error 5.1.8 Mean?
Enhanced status code 5.1.8 means the receiving server attempted to verify your sender domain and found it does not exist or is unresolvable. Many mail servers check that the sender domain has valid DNS records (MX or A records) as an anti-spam measure. If your sending domain cannot receive bounce messages, the server considers it suspicious.
This error requires you to ensure your sending domain has proper DNS configuration. At minimum, the domain should have valid MX records or an A record that allows bounce messages to be returned to you.
Common Causes
- Sender domain has no DNS records (not registered or expired)
- Sender domain has no MX records and no A record
- DNS for the sender domain is temporarily unresolvable
- Sending from a domain you do not control
How to Fix Error 5.1.8
- Verify your sending domain has valid MX records
- Ensure your domain registration has not expired
- Add proper DNS records (MX and A records) for your sending domain
- Use a domain that you own and control for sending
Frequently Asked Questions
Bounce code 5.1.8 means the sender's domain (the part after the "@" in the From address) does not exist or is incapable of accepting return mail. The recipient's mail server performed a DNS lookup on your sending domain and could not find valid MX or A records. This is a permanent failure that prevents delivery until the sender's domain DNS is properly configured.
Verify your sending domain's DNS records using a public DNS lookup tool. Ensure your domain has valid MX records pointing to a working mail server and an A record resolving to a valid IP address. Check that your domain registration has not expired and that DNS propagation is complete. If using a new domain, allow 24-48 hours for DNS records to fully propagate before sending email.
Error 5.1.7 indicates a syntax problem with the sender's mailbox address (the local part or overall formatting), while 5.1.8 specifically indicates the sender's domain is invalid or does not exist. In simple terms, 5.1.7 is about how the address is written (formatting), while 5.1.8 is about whether the domain behind the address actually exists and can receive reply mail. Both require sender-side fixes, but 5.1.8 specifically requires DNS/domain verification.