Enhanced Status Code 5.1.8: Bad Sender System Address

Permanent failure High severity Address RFC 3463

What it means

Enhanced Status Code 5.1.8 means “Bad Sender System Address.” 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.

At a glance

Code5.1.8

Bounce typeHard (permanent)

SeverityHigh

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 3463

What it looks like in your mail logs

host mx.example.net[203.0.113.40] said: 553 5.1.8 <noreply@brokensender.invalid>: Sender address rejected: Domain not found (in reply to MAIL FROM command)

What does 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.

How 5.1.8 plays out

Your server attempts delivery

The recipient server returns a permanent 5.1.8 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 5.1.8 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
5.1.8 is		✓ this code

Common causes of 5.1.8

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 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

What does bounce code 5.1.8 "Bad sender's system address" mean?

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.

How do I fix a 5.1.8 bounce error?

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.

What is the difference between 5.1.7 and 5.1.8?

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.

Related bounce codes

Sources & further reading

RFC 3463 · The standard that defines this reply code

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes