When your email can not be delivered, the SMTP server returns a notification, which includes an error message, associated with the actual problem.
Each code is composed of three digits (X.X.X). The first digit reflects the status of the email message as below:
2: the email was sent successfully
4: means there was a temporary problem when sending the email (the email server may try to re-send it, or you may have to do so. This will depend on your server settings). Such error messages display as 4.X.X, where X.X are used in order to give more precise information about the error;
5: means there is a permanent/fatal error related to the email (the email address of the receiver does not exist, it doesn't accept emails from you, etc.). Such error messages display as 5.X.X, where X.X are used in order to give more precise information about the error.
More information on SMTP Error Codes can be found on the link below, or you can copy and paste the URL into your web browser: