550: mailbox unavailable
The IONOS-hosted address you wrote to no longer exists, or the mailbox was temporarily deactivated after a long period without use.
- 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 Requested action not taken: Mailbox unavailable
What it means
This is the IONOS hard bounce for a recipient. The help page gives two reasons: the address no longer exists on IONOS systems, or it has been temporarily deactivated because nobody used it for a long time. Either way the message was not delivered and will not be retried. IONOS asks you to check the spelling of the address and whether it is still in use. If you send to a list, remove the address, since mail to unknown addresses is what gets senders blocked by IONOS (its 421 policy violations reply).
Common causes
- A typo in the address.
- The address was removed by its owner or administrator.
- The mailbox was deactivated after a long period without use.
How to fix it
- 01Check the spelling of the address.
- 02Confirm with the recipient, by other means, that the address is still in use.
- 03Remove the address from your lists if it stays undeliverable.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 550Mailbox does not exist or is disabled.The GMX, WEB.DE or mail.com address you wrote to does not exist, was disabled after a long period without use, or was blocked by the provider.
- 421IP blocked for mail to unknown addresses.IONOS has blocked your server IP from sending to its customers for now, because too much of your mail went 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.
Further reading
All 17 IONOS codesEvery code in the wikiPaste your own bounce