How to use it
- Copy the error text from the bounce (usually after “Diagnostic-Code” or “Remote server returned”), or paste the whole bounce.
- Read the recognised basic and enhanced codes and what they mean.
- Use “Usually whose problem” and “What to do” to troubleshoot; provider-specific codes get their own note.
How to read an SMTP bounce code
A bounce usually carries two codes: a three-digit basic code (RFC 5321, e.g. 550) and an x.y.z enhanced status code (RFC 3463, e.g. 5.1.1). The first digit is the outcome: 2 success, 4 temporary failure, 5 permanent failure. The enhanced code’s middle number is the area: 1 address, 2 mailbox, 3 mail system, 4 network and routing, 5 protocol, 6 content, 7 security and policy.
The same code can mean slightly different things at different providers, and the text after it is often more precise than the code itself, so read both.
Common SMTP codes
| Code | Meaning | Usually whose problem | What to do |
|---|---|---|---|
421 | Service not available; the server is closing the connection (often rate limiting or maintenance). | Network or temporary state | Wait for the automatic retry; if it keeps happening, slow down sending and check the IP’s reputation. |
450 | Mailbox temporarily unavailable (locked, busy or greylisted). | Recipient address or receiving server | A retry usually succeeds; greylisting lets the first retry through. |
451 | Local error in processing; the request was aborted. | Network or temporary state | Wait for the retry; with a 4.7.x code it is usually an anti-spam check on their side. |
452 | Insufficient storage, or too many recipients in one go. | Recipient address or receiving server | The mailbox may be full; for bulk mail, send to fewer recipients per message. |
500 | Syntax error; command not recognised. | Network or temporary state | Usually the sending software or a firewall altered the SMTP session; check the client or middlebox. |
501 | Syntax error in parameters (often a malformed sender or recipient address). | Sender configuration or reputation | Look for spaces, full-width characters or stray angle brackets in the address. |
503 | Bad sequence of commands (for example sending before authenticating). | Sender configuration or reputation | Turn on “SMTP server requires authentication” in the mail client. |
530 | Authentication required (or STARTTLS must be used first). | Sender configuration or reputation | Check the client’s SMTP username, password (or app password) and encryption setting. |
535 | Authentication failed: wrong username or password. | Sender configuration or reputation | QQ, 163, Gmail and others need an app password (authorisation code), not the login password. |
550 | Requested action not taken: mailbox unavailable (missing, disabled or refused by policy). | Recipient address or receiving server | Read it with the enhanced code: 5.1.x is an address problem, 5.7.x a policy or authentication problem. |
551 | User not local; try another path. | Recipient address or receiving server | Check that the recipient domain’s MX points at the right server. |
552 | Exceeded storage allocation or message too large. | Message content or size | Compress attachments or share them as a link; the recipient’s mailbox may also be full. |
553 | Mailbox name not allowed (bad address syntax or unauthorised sender address). | Sender configuration or reputation | Send from the address you log in with, or add it as an allowed alias in the provider’s console. |
554 | Transaction failed (often rejected as spam or the IP is blocked). | Sender configuration or reputation | Check the sending IP on blacklists, SPF/DKIM/DMARC and the content. |
4.2.2 | Recipient mailbox full (temporary). | Recipient address or receiving server | Wait for them to free up space or contact them another way. |
4.4.1 | No answer from the receiving host. | Network or temporary state | Usually an outage or network issue at their end; it retries automatically. |
4.4.7 | Message expired in the queue; delivery timed out. | Network or temporary state | Check that the recipient’s MX is correct and their server is reachable. |
4.7.0 | Temporary security or policy refusal (often throttling, TLS or reputation checks). | Sender configuration or reputation | Slow down, and make sure TLS and reverse DNS (PTR) are in place. |
5.1.1 | The recipient address does not exist. | Recipient address or receiving server | Check the spelling and remove the address from your lists. |
5.1.2 | The recipient’s domain is invalid or does not exist. | Recipient address or receiving server | Check the domain after @ and look up its MX records. |
5.1.8 | Sender address rejected (sender domain missing or unverifiable). | Sender configuration or reputation | Make sure the sender domain has valid MX or A records. |
5.1.10 | Recipient not found (Microsoft 365: recipient address lookup failed). | Recipient address or receiving server | Check the spelling; in your own tenant, check the mailbox exists and is licensed. |
5.2.1 | The recipient’s mailbox is disabled. | Recipient address or receiving server | Ask the recipient for a current address. |
5.2.2 | The recipient’s mailbox is full. | Recipient address or receiving server | Resend after they free up space, or contact them another way. |
5.3.4 | The message exceeds the server’s size limit. | Message content or size | Shrink attachments or send a link instead. |
5.4.1 | Recipient address rejected (Microsoft 365: Access denied, domain not accepted). | Recipient address or receiving server | The recipient’s admin should check accepted domains and that the recipient exists in Microsoft 365. |
5.4.4 | Unable to route: no mail server found for the recipient domain. | Network or temporary state | Look up the recipient domain’s MX records to see whether it accepts mail. |
5.7.1 | Refused for security or policy reasons (relay denied, blacklist, spam verdict and so on). | Sender configuration or reputation | Read the exact reason in the bounce: check blacklists and SPF/DKIM/DMARC, or whether the recipient only accepts certain senders. |
5.7.8 | Authentication credentials invalid. | Sender configuration or reputation | Update the SMTP password or app password. |
5.7.23 | SPF validation failed; the sending server is not in the sender domain’s SPF record. | Sender configuration or reputation | Add the sending service to your SPF record. |
5.7.25 | Gmail: the sending IP has no PTR (reverse DNS) record. | Sender configuration or reputation | Ask your server or IP provider to set a PTR that matches your HELO host name. |
5.7.26 | Gmail: the message is unauthenticated (SPF and DKIM both failed, or DMARC policy rejected it). | Sender configuration or reputation | Set up SPF and DKIM and make sure they align with the From domain. |
5.7.27 | Sender domain does not accept mail (null MX) or the sender address is invalid. | Sender configuration or reputation | Send from an address that can receive replies. |
5.7.57 | Microsoft 365: the client tried to send anonymously without authenticating. | Sender configuration or reputation | Enable SMTP authentication in the device or app (smtp.office365.com:587 with STARTTLS). |
5.7.509 | Microsoft 365: the message failed DMARC and the sender domain’s policy is reject. | Sender configuration or reputation | Fix SPF/DKIM alignment, or check whether forwarding broke it. |
5.7.511 | Microsoft 365: the sending IP is banned by Microsoft. | Sender configuration or reputation | Follow the bounce instructions and forward the full bounce to delist@microsoft.com. |
5.7.606 | Microsoft 365: the sending IP is on Microsoft’s block list. | Sender configuration or reputation | Request delisting through the sender.office.com portal. |
5.7.708 | Microsoft 365: traffic from a low-reputation IP range is not accepted. | Sender configuration or reputation | Switch to a reputable sending service or a dedicated IP. |
4.7.28 | Gmail: unusual volume from this IP or domain; temporarily rate limited. | Sender configuration or reputation | Slow down, ramp volume gradually and check for compromised accounts. |
Frequently asked questions
What does 550 5.1.1 mean?
The recipient address does not exist. Check for typos or whether the person has left or closed the mailbox; remove such addresses from mailing lists promptly to protect your sender reputation.
Do I need to resend after a 4xx error?
Usually not: the sending server retries from its queue, typically for 1–5 days. A “still trying” delay notice can be left alone; act if it eventually turns into a 5xx.
The bounce says my mail was rejected as spam (554 / 5.7.1). What now?
Check that SPF, DKIM and DMARC pass and that the sending IP is not blacklisted, then review the content for shortened links, heavy images or attachments. After fixing, use the provider’s delisting or support form.
Can I get the explanation in my own language?
Yes. Paste the original text and each code is explained in this page’s language, with the original wording kept alongside for reference.