Email / SMTP Response and Error Numbers
Simple Mail Transfer Protocol i.e., SMTP is used for sending and receiving emails. Whenever an email is rejected by the third party server or by the parent email server, then SMTP produces a specific Error/Response, which can be used to troubleshoot/identify the cause of email delivery failure. We gathered most of the SMTP errors/responses users most likely encounter when performing email practises. Going further, we also included the Error/Response description and possibly responsible for the SMTP error/response and that too covered individually. You can click on the Error/Response Code or the Resolution link to learn more about the error and solution (if available) individually.
| SMTP Response Number | Definition | How to Resolve / Examples |
| 101 | Cannot open connection | How to fix SMTP Error 101 |
| Examples: | “SMTP Error 101, Error opening connection” | |
| Examples: | “SMTP Error 101, cannot open SMTP stream” | |
| 111 | Connection refused or inability to open an SMTP stream | How to fix SMTP Error 111 |
| 211 | System status message or system help reply | How to fix SMTP Error 211 |
| 214 | Help message (Information on non-standard command) | How to fix SMTP Error 214 |
| 220 | <SERVER> Service ready | How to fix SMTP Error 220 |
| 221 | <SERVER> Service closing transmission channel | How to fix SMTP Error 221 |
| Examples: | “221 Closing connection” | |
| Examples: | “221 Goodbye” | |
| 250 | Requested mail action okay, completed | How to fix SMTP Error 250 |
| 251 | User not local; will forward to <REMOTEPATH> | How to fix SMTP Error 251 |
| 252 | Can not verify user; accepting message for attempt | How to fix SMTP Error 252 |
| 354 | Starting mail input | How to fix SMTP Error 354 |
| 420 | Timeout connection problem | How to fix SMTP Error 420 |
| NOTE: | This error is related to GroupWise Servers only. | |
| 421 | <SERVER> Service not available, closing transmission channel | How to fix SMTP Error 421 |
| 431 | Not enough storage or out of memory (See #452, #552) | How to fix SMTP Error 431 |
| 432 | Recipients Exchange Server incoming mail queue stopped | How to fix SMTP Error 432 |
| NOTE: | This error is related to Microsoft Exchange Servers only. | |
| 441 | Recipients mail server is not responding | How to fix SMTP Error 441 |
| 442 | Connection dropped during transmission | How to fix SMTP Error 442 |
| 446 | The maximum hop count was exceeded for the message: an internal loop has occurred. | How to fix SMTP Error 446 |
| 447 | Your outgoing message timed out because of issues concerning the incoming server. | How to fix SMTP Error 447 |
| 449 | A routing error. | How to fix SMTP Error 449 |
| NOTE: | This error is related to Microsoft Exchange Servers only. | |
| 450 | Requested mail action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons) | How to fix SMTP Error 450 |
| Examples: | “450 Please try again later” | |
| “SMTP Error 450 5.2.3 Msg Size greater than allowed by Remote Host” | ||
| 451 | Requested action aborted: error in processing | How to fix SMTP Error 451 |
| Examples: | “SMTP error 451 Unable to complete command, DNS not available or timed out” | |
| “451 Domain of sender address does not resolve” | ||
| 452 | Requested action not taken: insufficient system storage | How to fix SMTP Error 452 |
| Example: | “452 Out of memory” | |
| 455 | Server unable to accommodate parameters | How to fix SMTP Error 455 |
| 471 | An error of your mail server, often due to an issue of the local anti-spam filter | How to fix SMTP Error 471 |
| 500 | Syntax error, command unrecognized (This may include errors such as command line too long) | How to fix SMTP Error 500 |
| 501 | Syntax error in parameters or arguments | How to fix SMTP Error 501 |
| Example: | “Remote host said: 501 Invalid Address” | |
| 502 | Command not implemented | How to fix SMTP Error 502 |
| 503 | Bad sequence of commands | How to fix SMTP Error 503 |
| 504 | Command parameter not implemented | How to fix SMTP Error 504 |
| 510 | Invalid email address | How to fix SMTP Error 510 |
| 511 | Invalid email address (See #552) | How to fix SMTP Error 511 |
| 512 | A DNS error: the host server for the recipient’s domain name cannot be found. | How to fix SMTP Error 512 |
| Example: | “SMTP Error 550 5.1.2 Host unknown – host cannot be found” | |
| 513 | Address type is incorrect (possibly misspelled) | How to fix SMTP Error 513 |
| Examples: | “SMTP error 501 5.1.3 Invalid address” | |
| Examples: | “SMTP error 553 5.1.3 User address required !” | |
| Examples: | “SMTP error 501 5.1.3 Bad recipient address syntax” | |
| 521 | <DOMAIN> does not accept mail | How to fix SMTP Error 521 |
| Example: | “521 5.2.1 : (CON:B1) blog.mailchannels.com/2012/09/why-did-i-get-521-5-2-1-conb1-httppostmaster-info-aol-comerrors554conb1-html“ | |
| 523 | The total size of your mailing exceeds the recipient server’s limits | How to fix SMTP Error 523 |
| Examples: | “SMTP Error 450 5.2.3 Message Size greater than allowed by Remote Host” | |
| Examples: | “SMTP Error 552 5.2.3 Data size exceeds maximum permitted” | |
| Examples: | “SMTP Error 552 5.2.3 Message exceeds maximum fixed size” | |
| 530 | Normally, an authentication problem. But sometimes it’s about the recipient’s server blacklisting yours, or an invalid email address. | How to fix SMTP Error 530 |
| 541 | The recipient address rejected your message: normally, it’s an error caused by an anti-spam filter. | How to fix SMTP Error 541 |
| 550 | Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons) | How to fix SMTP Error 550 |
| Examples: | “550 Invalid recipient” | |
| Examples: | “550 User account is unavailable” | |
| Examples: | “550 No such user here” | |
| 551 | User not local; please try <FORWARDPATH> | How to fix SMTP Error 551 |
| 552 | Requested mail action aborted: exceeded storage allocation | How to fix SMTP Error 552 |
| Example: | “552 sorry, mailbox user@somecompany.com is over quota temporarily (#5.1.1)” | |
| 553 | Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect) | How to fix SMTP Error 553 |
| 554 | Transaction failed (Or, in the case of a connection-opening response, “No SMTP service here”) | How to fix SMTP Error 554 |
| 555 | MAIL FROM/RCPT TO parameters not recognized or not implemented | How to fix SMTP Error 555 |
(Ref: SMTP Protocol RFC )