How many times you got your outgoing emails back and wondered what is wrong with it? Every time when your email can not be delivered, the SMTP server sends you a notification, which includes a standard error message, associated with the real problem.
Each code is composed of three digits (X.X.X). The first digit gives the status of the email message:
2 means the email was succesfully sent;
4 means there was a temporary problem while sending the email (your email server may try to send it again or you have to resend it, depending on your server settings). Such error messages are using codes like 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 are using codes like 5.X.X, where X.X are used in order to give more precise information about the error.
Here is a complete list of email delivery error codes, based on the Extended SMTP (ESMTP) standards, where X can be 4 or 5, depending on the error type (Persistent Transient or Permanent):