Simple Mail Transfer Protocol (SMTP) is the most popular protocol for transferring emails. Sometimes, after sending an email, you may receive an automated delivery failure notice mentioning SMTP error codes with numbers like 421 or 550. This article will explain what these error codes mean and how you can resolve them.
What is SMTP?
The Simple Mail Transfer Protocol is a set of rules that mail servers and clients use to communicate. Whenever you click the send button, you transfer the message’s contents from one mail server to another. The SMTP server application transmits the message and ensures it gets to the intended recipient’s server.
How does it Work?
Here’s how this system works to transfer a message:
- Your email client connects to the SMTP server you specified during the initial setup. This process is called a handshake.
- The server receives the sender’s and recipient’s email addresses and the message contents provided by the client.
- If the sender’s and recipient’s addresses are on the same domain, it forwards the message to the target mailbox. If not, it queries the Domain Name Service (DNS) to find the IP address of the recipient’s email server. Once it knows the correct IP address, it sends the message to that SMTP server.
- The receiving SMTP server verifies incoming email and puts it into the client’s mailbox.
SMTP Errors
The SMTP protocol uses numeric codes to indicate the status of an email transfer. If your mail is not delivered, you’ll receive an error message with a code indicating the problem. SMTP error codes start with a 4 or 5, followed by two more digits. 4 indicates a temporary failure, in which case the originating SMTP server will automatically resend the message. 5 signifies a permanent failure that needs additional actions to fix.
Common error codes include:
| 421 | Email service is unavailable; try again later. |
| 450 | The mailbox is currently unreachable. |
| 451 | Temporary server problems. |
| 452 | Insufficient storage space on the receiving server; you may ask the recipient to free up space. |
| 455 | Need help to process command. |
| 500 | Unable to process command due to a syntax error. |
| 502 | The server understands the command but is unable to perform it. |
| 503 | Bad sequence of commands. |
| 541 | Message rejected by the recipient. It may occur due to filtering configuration or if the anti-spam software marks the message as spam. |
| 550 | The recipient’s email address is unavailable. |
| 552 | Insufficient storage space on the recipient’s server. |
| 554 | Command failed with no additional details. |
The second digit of an error code may give you an extra hint about the issue. It should be noted, however, that this convention is often used inconsistently.
- 0 is a syntax error. It usually shows up when your server sends an invalid command or a command with a structural error.
- 2 indicates a failure due to security or hardware issues. It usually auto-resolves on further attempts.
- 3 and 4 are unspecified errors.
- 5 signifies that the SMTP system itself is responsible for the problem.
Troubleshooting SMTP Errors
Software misconfiguration is a popular cause of SMTP errors. The first thing to check your client’s settings is checking your client’s settings. If the problem persists, you’ll have to contact the support team to resolve the issue.
A more widespread cause of failure is trying to send an email to a non-existent address. Reach your contact by alternative means to verify if their address is still working and check whether you spell it correctly.
Finally, you may be sending an email that looks like spam or has attachments that can be considered malicious. In this case, try to contact the email administrator of the addressee’s domain, usually available at [email protected].
Conclusion
We have explained the common SMTP error codes you may encounter while sending emails. At this point, you should be able to decipher error codes and resolve them.
