IONOS (1&1) · permanent error
550: sender 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.
- IONOS
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550
- Provider
- IONOS (1&1)
- 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, defined in RFC 7505, is an MX record whose target is a single dot. It declares that the domain receives no email. IONOS rejects mail from domains that publish one, and this 550 means your sender domain does. The IONOS fix is to delete the No Service MX entry in the DNS settings of your domain, and in practice to replace it with an MX record for the server that receives your mail.
Common causes
- The domain was set up as a no-mail domain and is now used to send.
- A parked-domain or web-only DNS template added the null MX.
How to fix it
- 01Check the MX records of your sender domain.
- 02Delete the null MX entry in your DNS settings.
- 03Add an MX record for the server that receives mail for the domain.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 550Sender 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.
- 556Recipient domain publishes a null MX.The domain you are writing to has a No Service MX record (null MX), which means it can neither receive nor send email, so the message cannot be delivered.
- 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 17 IONOS codesEvery code in the wikiPaste your own bounce