550: sender domain publishes a null MX
GMX refused the mail because your sender domain publishes a null MX record, which says the domain accepts no email. GMX only takes mail from domains that can receive replies.
- GMX, WEB.DE
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550
- 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 sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 Sender address has null MX
What it means
A null MX (RFC 7505) is an MX record with a single dot as its target. It tells the world that a domain receives no mail at all. Case r1402 means your sender domain has one. GMX only accepts mail when it is possible to send a message back to the sender domain, so it refuses yours with 550. Domains often get a null MX when they are parked or meant only for websites, and then someone starts sending from them. The domain owner has to replace it with a real MX record.
Common causes
- The domain was set up as a no-mail domain and is now used to send.
- A DNS template for parked or web-only domains added the null MX.
- The sender address uses the wrong domain or subdomain.
How to fix it
- 01Check the MX records of the domain in your sender address.
- 02Replace the null MX with an MX record for a server that accepts mail for the domain.
- 03If the domain is not meant for mail, send from a domain that is.
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 r1402: 550 Sender address has null MXpostmaster.gmx.net
Related codes
- 556Recipient 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.
- 550Sender domain publishes a null MX.Your sender domain publishes a No Service MX record (null MX), which says it accepts no mail, and IONOS rejects mail from such domains.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
550 at other providers
- 550 [SC-001]Outlook.com policy block for content or reputation.Outlook.com rejected the mail for policy reasons tied to spam-like content or to the reputation of the sending IP or domain.
- 550 [SC-002]Outlook.com block for namespace mining.Outlook.com blocked the sending IP because it behaved like a namespace miner, checking whether addresses exist without sending mail.
- 550 [SC-003]Outlook.com block for an open proxy or relay.Outlook.com rejected the mail because the sending IP appears to be an open proxy or open relay.
- 550 [SC-004]Outlook.com block after complaints.Outlook.com placed a block on the sending IP because it received complaints about mail coming from it.
- 550 [DY-001]Outlook.com refuses dynamic IP addresses.Outlook.com rejected the mail because the sending IP is dynamic address space, which it generally does not accept mail from.
- 550 [DY-002]Outlook.com block for a likely compromised sender.Outlook.com rejected the mail because the sending server or computer appears to be compromised or infected.
Further reading
All 57 GMX, WEB.DE codesEvery code in the wikiPaste your own bounce