SMTP Error 556: Domain Does Not Accept Mail

Permanent failure Critical severity Address RFC 7505

What it means

SMTP Error 556 means “Domain Does Not Accept Mail.” The recipient domain has explicitly declared it does not accept any email by publishing a null MX record. This is a permanent failure - the domain cannot receive email and the address should be removed from your list immediately.

At a glance

Code556

Bounce typeHard (permanent)

SeverityCritical

CategoryAddress

What to doSuppress the address; do not retry

StandardRFC 7505

What it looks like in your mail logs

556 5.1.10 Domain example.com does not accept mail (nullMX)

What does 556 mean?

SMTP code 556 indicates that the recipient domain has published a null MX record, explicitly declaring that it does not accept email. This is defined in RFC 7505 and means the domain is intentionally configured to not receive any email.

A null MX record is a single MX record with a priority of 0 and a target of "." (a single period). This is used by domains that exist for web services, CDNs, or other non-email purposes. There is absolutely no way to deliver email to these domains. Remove any addresses at these domains from your list immediately.

How 556 plays out

Your server attempts delivery

The recipient server returns a permanent 556 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 556 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
556 is		✓ this code

Common causes of 556

Domain has published a null MX record (RFC 7505)
Domain is used exclusively for web services, not email
Organization has deprecated email for this domain

How to fix 556

Remove the address from your mailing list permanently
Verify using an MX Lookup tool that the domain has no mail servers
Find an alternative contact domain for the recipient

Frequently asked questions

What does SMTP error 556 mean?

SMTP error 556 means "Domain does not accept mail." The recipient's domain has been explicitly configured with a Null MX record, which is a DNS entry declaring that the domain cannot receive email. This is a permanent rejection defined in RFC 7505, and the message cannot be delivered to any address at that domain. No amount of retrying will resolve this error.

What is a Null MX record?

A Null MX record is a special DNS entry (defined in RFC 7505) that explicitly declares a domain does not accept email. When a domain publishes a Null MX, any mail server attempting to deliver email to that domain will immediately reject the message with a 556 error instead of queuing it for retry. Organizations use Null MX for domains that are not intended to receive email, providing an instant delivery failure notification rather than delayed bounces.

How do I fix SMTP error 556?

As a sender, you cannot fix SMTP 556 because the recipient's domain is intentionally configured to reject all incoming email. Verify the email address is correct and that you are sending to the right domain. If the domain should accept email, contact the domain administrator to remove the Null MX record and configure proper MX records. If you control the domain, replace the Null MX entry with a valid MX record pointing to your mail server.

Related bounce codes

Sources & further reading

RFC 7505 · The standard that defines this reply code RFC 7504: SMTP 521 and 556 Reply Codes · Defines reply code 556 for domains that do not accept mail RFC 7505: A "Null MX" No Service Resource Record · Defines the null MX record and the 5.1.10 enhanced status code

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