550: sender domain has no valid MX or A record
GMX refused the mail because your sender domain is not set up in DNS as the RFCs require: it needs an MX record whose host has a valid A record.
- 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 invalid DNS MX or A/AAAA resource record
What it means
GMX only accepts mail from domains that could receive a reply. Case r1401 means the sender domain is not configured in line with the RFCs: GMX expects an MX record pointing to a host that itself has a valid A record (RFC 5321 section 5.1, RFC 2181 section 10.3, which also rules out MX targets that are aliases). Without that, a GMX user could not answer you, so the mail is refused with 550. The fix is in your DNS. GMX also warns that it cannot reply to a domain that is misconfigured in this way, so give an address on another domain if you write to it.
Common causes
- The domain has no MX record and no A record.
- The MX record points to a host name that does not resolve.
- The MX record points to a CNAME alias or directly to an IP address.
How to fix it
- 01Look up the MX records of your sender domain and the A record of each MX host.
- 02Publish an MX record that points to a host name with a valid A record.
- 03Replace MX targets that are CNAMEs or bare IP addresses.
- 04If you need to contact GMX before DNS is fixed, use an address on another domain.
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 r1401: 550 invalid DNS MX or A/AAAA resource recordpostmaster.gmx.net
Related codes
- 550Recipient domain has no valid MX or A record.The recipient's domain is not set up in DNS as the RFCs require, with an MX record whose host has a valid A record, so GMX refused the mail.
- 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.
- 550Sender domain has no valid MX or A record.IONOS refused the mail because your sender domain is not set up in DNS as the RFCs require, so a reply to it could not 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 57 GMX, WEB.DE codesEvery code in the wikiPaste your own bounce