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.
What Does Error 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.
Common Causes
- 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 Error 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
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.
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.
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.