Email Delivery Codes - Errors reported by Email applications

Have you ever had 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):
X.1.0 Other address status
X.1.1 Bad destination mailbox address
X.2.0 Bad destination system address
X.1.3 Bad destination mailbox address syntax
X.1.4 Destination mailbox address ambiguous
X.1.5 Destination mailbox address valid
X.1.6 Mailbox has moved
X.1.7 Bad sender's mailbox address syntax
X.1.8 Bad sender's system address
X.2.0 Other or undefined mailbox status
X.2.1 Mailbox disabled, not accepting messages
X.2.2 Mailbox full
X.2.3 Message length exceeds administrative limit.
X.2.4 Mailing list expansion problem
X.3.0 Other or undefined mail system status
X.3.1 Mail system full
X.3.2 System not accepting network messages
X.3.3 System not capable of selected features
X.3.4 Message too big for system
X.4.0 Other or undefined network or routing status
X.4.1 No answer from host
X.4.2 Bad connection
X.4.3 Routing server failure
X.4.4 Unable to route
X.4.5 Network congestion
X.4.6 Routing loop detected
X.4.7 Delivery time expired
X.5.0 Other or undefined protocol status
X.5.1 Invalid command
X.5.2 Syntax error
X.5.3 Too many recipients
X.5.4 Invalid command arguments
X.5.5 Wrong protocol version
X.6.0 Other or undefined media error
X.6.1 Media not supported
X.6.2 Conversion required and prohibited
X.6.3 Conversion required but not supported
X.6.4 Conversion with loss performed
X.6.5 Conversion failed
X.7.0 Other or undefined security status
X.7.1 Delivery not authorized, message refused
X.7.2 Mailing list expansion prohibited
X.7.3 Security conversion required but not possible
X.7.4 Security features not supported
X.7.5 Cryptographic failure
X.7.6 Cryptographic algorithm not supported
X.7.7 Message integrity failure