550: spam domain or invalid HELO
IONOS refused the mail because your sender domain is known as a spam domain, or because your server HELO/EHLO is not a fully qualified domain name.
- 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 Reject due to policy restrictions
What it means
IONOS uses this 550 for two different problems. One is reputation: the sender domain is known to IONOS as a spam domain, which it suggests checking with a blocklist provider such as Spamhaus and having the entry removed. The other is configuration: the HELO/EHLO name of the sending server is not a fully qualified domain name (FQDN), for example a bare host name or localhost. The bounce does not say which applies, so check both. If the error remains after both are in order, IONOS asks you to contact it.
Common causes
- Your sender domain is listed as a spam domain, for example on a Spamhaus list.
- Your server announces a HELO/EHLO name that is not a fully qualified domain name.
How to fix it
- 01Check your sender domain with Spamhaus and other blocklists, and request removal if it is listed.
- 02Set your HELO/EHLO to a fully qualified host name, such as mail.yourdomain.tld.
- 03If the rejection continues after the fix, contact IONOS through its postmaster contact form (postmaster-contact.ionos.com), which its SMTP error messages page links to, quoting the full error.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 554Rejected under IONOS policy.IONOS refused the mail because it violates IONOS policy, most often because the sending server is mainly sending spam.
- 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.
- 550Sender domain on a Spamhaus blocklist.GMX refused your mail because your envelope or From domain is listed on a Spamhaus blocklist. GMX makes no exceptions; the listing must be removed at Spamhaus.
- 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