556: recipient domain publishes a null MX
The domain you are writing to publishes a null MX record, which declares that it accepts no email, so the message cannot be delivered.
- GMX, WEB.DE
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 556
- Provider
- GMX, WEB.DE and mail.com
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The recipient's side. Their mail admin has to change something; you can tell them.
- What the server says
556 Recipient address has null MX
What it means
A null MX (RFC 7505) is the way a domain says it does not receive mail. Case r1403 means the recipient domain publishes one, and GMX answers with 556, the reply RFC 7504 defines for a domain that does not accept mail. No retry will change this: the message can only be delivered if the domain owner replaces the null MX with an MX record for a server that accepts mail. If you believe the address is right, tell the recipient through another channel.
Common causes
- The recipient domain was deliberately set up to receive no mail.
- A parked or web-only domain template added a null MX.
- The address uses the wrong domain, for example a brand domain that is not used for email.
How to fix it
- 01Check that you have the right domain for the recipient.
- 02Ask the recipient for an address on a domain that accepts mail.
- 03If you own the domain, replace the null MX with an MX record for your mail server.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- GMX Postmaster case r1403: 556 Recipient address has null MXpostmaster.gmx.net
Related codes
556 at other providers
Further reading
All 57 GMX, WEB.DE codesEvery code in the wikiPaste your own bounce