Guide to SMTP Error Codes and How to Address Them

Overview of SMTP Error Codes

SMTP, short for Simple Mail Transfer Protocol, is the foundation for transmitting emails over the Internet. During the process of sending emails, it's common to encounter certain error codes. These codes are responses from the recipient's email server, signifying that the message could not be delivered. Familiarizing yourself with these error codes is essential for identifying and fixing email delivery problems.

Frequently Occurring SMTP Error Codes and Their Resolutions

Below are some common SMTP error codes, explanations of what they mean, and suggested actions to resolve them:

1. Error Codes for Mailgun SMTP
400 Series Errors (Bad Requests):

• Missing from/to/message parameter: Make sure both the sender's and recipient's email addresses are included, along with the body of the email.

• Missing 'text' or 'html' parameters: Ensure your email contains either a plain text version or an HTML version.

• Too many recipients; limit is 1000: Reduce the number of email recipients to 1000 or fewer.

• 401 Forbidden: The user lacks necessary permissions. Review and confirm the required permissions for the request.

• 404 Domain not found: Ensure you're using a valid domain that exists.

• 429 Request limit exceeded: Wait for the specified cooldown period before resubmitting the request.

• 500 Internal Server Error: Retry sending the request later or reach out to the server administrator for help.
  
  

2. Error Codes for Gmail/Google Workspace

• 421, "4.3.0" Temporary system issue: Try sending the email again after some time.

• 450, "4.2.1" Recipient receiving too many emails: Wait a while and attempt to resend the email later.

• 451, "4.3.0" Temporary rejection by mail server: Try resending the email later.

• 550, "5.1.1" Invalid recipient email address: Double-check the recipient's email for mistakes or typos.

• 550, "5.4.5" Exceeded daily sending limit: Reduce the number of emails being sent daily or consider upgrading your Google Workspace plan.

Troubleshooting and Frequently Asked Questions

• What if the error doesn’t go away?
  If an issue persists despite applying the suggested fix, it may be time to contact your email provider for further assistance.
  
  

• How can I prevent SMTP errors?
  Regularly validate email addresses before sending, follow best practices for email delivery, and keep an eye on your email reputation to minimize errors.
  
  

• Can I resend an email after encountering an SMTP error?
  Yes, but first ensure you understand the error code and apply the corresponding solution. For temporary errors like a server being down, trying again after some time might succeed.