GMX, WEB.DE and mail.com · permanent error
550: HELO name changes or fails reverse lookup
GMX refused the mail because your server announces different HELO names from one connection to the next, or the reverse lookup of its HELO name failed.
- 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 Reject due to policy restrictions
What it means
A legitimate mail server introduces itself the same way every time. Case r1001 means GMX saw your server use several different HELO names, or could not confirm the HELO name with a reverse lookup, and refused the message with 550. The case page asks you to stop the alternating HELO and to make sure the HELO/EHLO meets the requirements of RFC 5321, which means a fully qualified host name.
Common causes
- Several servers or containers behind one IP announce different names.
- The HELO name is generated at random or taken from a changing host name.
- The HELO name does not match the reverse DNS of the sending IP.
- Malware on a machine in your network sends mail with its own made-up names.
How to fix it
- 01Set one fixed, fully qualified HELO name for each sending IP.
- 02Make the HELO name, its A record and the PTR record of the IP agree.
- 03Check the network for machines sending mail directly that should not.
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 r1001: 550 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 554HELO name changes or fails reverse lookup (554).GMX refused the mail with a 554 because your server uses alternating HELO names, or its HELO name failed the reverse lookup.
- 550HELO name not accepted.GMX refused your mail because the HELO name your server announces is not a resolvable host name, or is on the GMX internal HELO blocklist.
- 450Deferred for a changing HELO or unknown addresses.IONOS delayed your mail because your server keeps changing its HELO/EHLO name, or because it sent too much to addresses that do not exist.
- 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.
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