SMTP Error Codes Explained: 421, 450, 535, 550, 552, 554
An SMTP error code is the mail server telling your mail app why it did not take your message. The first digit is the headline: a code starting with 4 means try again later, and a code starting with 5 means it will not work until something changes. Read the number, find it below, and you will know whether to wait, fix a setting or ask the recipient.
Last reviewed: 11 November 2026, against the SMTP standards (RFC 5321, RFC 3463 and RFC 4954).
Before you start
- The full bounce message. It carries the three-digit code, often a code like
5.1.1, and the server's own words. Copy all of it. - Which side the error comes from. Your own mail server rejects you while you send (a code during sending, often 530 or 535). The recipient's server rejects the message after you have sent it (a bounce that arrives later, often 550 or 554).
How do you read a code?
| Part | Meaning | Example |
|---|---|---|
| First digit | 4 is temporary, 5 is permanent | 4xx: the server will retry. 5xx: it will not |
| Three-digit reply code | What the server did | 550 |
| Enhanced status code | Class, subject and detail | 5.1.1: permanent, address, bad mailbox |
The enhanced code, such as 5.7.1, is usually more helpful than the three-digit one.
Which codes mean wait, and which mean fix?
Temporary: 4xx
| Code | Meaning | What to do |
|---|---|---|
| 421 | The server is not available or is closing the connection | Wait and send again. The server is busy or restarting |
| 450 | The mailbox is unavailable for now | Wait. Check later if it persists |
| 451 | The server hit an error processing the message | Wait. Often a temporary block while the receiving server checks you |
| 452 | Not enough storage on the server, or too many recipients | Wait, or send to fewer recipients at once |
| 454 | Authentication is failing for now | Try again, then check the password |
Mail servers retry 4xx messages by themselves, often for several days. You do nothing unless you get a final failure notice.
Permanent: 5xx
| Code | Meaning | What to do |
|---|---|---|
| 500, 501 | The command or its arguments were not understood | Check the server settings in your app. Update the app |
| 503 | Commands arrived in the wrong order | Check the connection security (SSL/TLS) setting |
| 530 | Authentication required | Turn on authentication for the outgoing server in your app and sign in with your full address and password |
| 535 | Authentication credentials invalid | The user name must be the full address. Check the password |
| 550 | Mailbox unavailable, or refused by policy | See the table below |
| 551 | The user is not local | The server wants another address. Check the recipient |
| 552 | Storage allocation exceeded, or the message is too large | Send a smaller message, or ask the recipient to free space |
| 553 | The mailbox name is not allowed | Check the address for a typing mistake |
| 554 | The transaction failed, often a block | See the table below |
What do the common 550 and 554 messages mean?
| Message | Meaning | Fix |
|---|---|---|
| 550 5.1.1, "user unknown" or "mailbox does not exist" | The address to the left of the @ is wrong |
Check the spelling, or ask the recipient |
| 550 5.1.2, "domain not found" | The domain to the right of the @ has no mail |
Check the domain spelling |
| 550 5.7.1, "relay access denied" or "relay not permitted" | The server will not send your message on, because you are not signed in | Turn on authentication in your app and use port 465 with SSL/TLS. See the Noiz mail settings |
| 550 5.7.1 from the recipient, "rejected as spam" or "blocked" | The recipient's server did not trust the message | Check SPF, DKIM and DMARC. Why your email goes to spam explains how |
| 552 5.2.2, "mailbox full" | The recipient's mailbox is full | Tell the recipient by another route |
| 552 5.2.3, "message too large" | The message or an attachment is over a limit | Send a link instead of a large file |
| 554, "transaction failed" or "no SMTP service here" | The server refused the whole message, often for reputation or content | Check the message for links and attachments, and the sender's authentication |
The server's wording varies, so rely on the numbers.
What are the most common fixes on a Noiz mailbox?
- Use the full address as the user name for sending as well as receiving.
- Use
mail.yourdomain.com, port465, SSL/TLS and authentication on for sending. The settings for each app are in one table. - Send a smaller message, or fewer recipients at once. Outgoing mail on shared hosting is subject to server limits on message size, recipients and connections, set out in the Acceptable Use Policy. If you need to send more, ask us first.
- Never send unsolicited bulk email. It is prohibited by the Acceptable Use Policy, and it is the quickest way to land on blocklists.
- Check your sender authentication, SPF, DKIM and DMARC, if a recipient rejects your mail.
Need a hand?
Open a ticket in the console and paste the whole bounce message, including the headers if you can. Never send your password.
Official Documentation Reference
- RFC 5321: Simple Mail Transfer Protocol: the reply codes.
- RFC 3463: Enhanced mail system status codes: the codes like 5.1.1.
- RFC 4954: SMTP service extension for authentication: the 530 and 535 codes.
