When your email cannot be delivered, the SMTP server generates a notification containing an error message that indicates the specific issue.
Each code consists of three digits (X.X.X), with the first digit representing the status of the email message as follows:
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2: Indicates that the email was sent successfully.
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4: Signifies a temporary problem when sending the email. In this case, the email server may attempt to resend it, or you may need to do so manually, depending on your server settings. These error messages will appear as 4.X.X, where X.X provides additional details about the error.
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5: Indicates a permanent or fatal error related to the email. This could mean that the recipient's email address does not exist or that it is not accepting emails from you. Such error messages are formatted as 5.X.X, where X.X offers more specific information about the issue.
For more information on SMTP Error Codes, you can visit the following link or copy and paste the URL into your web browser:
https://wiki.mattrude.com/SMTP_Status_Codes