550: mailbox 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
550 Requested action not taken: mailbox unavailable
What it means
550 is the most common permanent rejection. RFC 5321's own examples are a mailbox not found, no access, and a command rejected for policy reasons, so the reply code alone does not say which applies. The enhanced code and text that follow do: 5.1.1 means the address does not exist, 5.7.1 a policy refusal, and the 5.7.20 to 5.7.26 codes failed authentication checks. Because it is a 5xx reply, the message is returned to the sender. An unknown-address 550 calls for removing the address; a policy 550 calls for fixing what the text names, such as authentication or a blocklist listing.
Common causes
- The recipient address does not exist (5.1.1).
- The receiver's policy refused the message, for example over a blocklist listing or failed SPF, DKIM or DMARC (5.7.x).
- The server will not relay for you, for example because you sent without logging in.
- The recipient's mailbox has been disabled.
How to fix it
- 01Read the enhanced code and the text after 550; together they give the reason.
- 02For an unknown address, check the spelling and remove the address from your list if it is wrong.
- 03For a policy refusal, check SPF, DKIM and DMARC for your domain and look up your IP on blocklists.
- 04If the reason is unclear, send the full bounce to the recipient's administrator.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
- RFC 5321, section 3.3: Mail Transactionsrfc-editor.org
Related codes
- 5.1.1Mailbox does not exist.The recipient's domain accepts mail, but the mailbox named before the @ does not exist there, so the message was refused and will not be delivered.
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
- 550 5.7.26Multiple authentication checks failed.The message failed more than one authentication check, for example SPF and DKIM, and the receiving server's policy rejects it.
- 550 5.7.23SPF check failed.The SPF check on the sending server's IP address gave a fail result for the sender's domain, and the receiving server's policy rejects such mail.
- 450Mailbox temporarily unavailable.The server did not take the message for this mailbox for now, because the mailbox is busy or temporarily blocked by policy. The sending server will try again.
- 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.
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 105 standard codesEvery code in the wikiPaste your own bounce