550: sender 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.
- 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 Invalid DNS MX or A/AAAA resource record
What it means
IONOS checks that the sender domain could receive mail in return. This 550 means it is not configured in line with the RFCs, and the help page lists the faults it looks for: the domain has neither an A nor an MX record; the MX record points to a name that cannot be resolved; the MX record points directly to an IP address; or the MX host has only a CNAME record instead of a valid A record. Any one of them is enough. Fix the record in your DNS and the mail is accepted again.
Common causes
- The sender domain has neither an MX nor an A record.
- The MX record names a host that does not resolve.
- The MX record contains an IP address instead of a host name.
- The MX host is a CNAME alias without its own A record.
How to fix it
- 01Look up the MX records of your sender domain and the A record of each MX host.
- 02Point the MX record to a host name that has a valid A record.
- 03Replace IP addresses and CNAME targets in MX records with proper host names.
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 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.
- 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 17 IONOS codesEvery code in the wikiPaste your own bounce