Bounce error codes and meanings

Bounce is an automatic response that you receive from the receiver email server as soon as you send the email. Generally, you will receive this response when the corresponding email address is invalid or disabled.

For details: What are the Bounce Addresses?

You can check over some error codes and possible causes in below.

Invalid or disabled email address

421 Service not available, closing transmission channel
450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451 Requested action aborted: error in processing
550 User’s mailbox was unavailable (such as not found)
551 The recipient is not local to the server.
553 The command was aborted because the mailbox name is invalid.
5.0.0 Address does not exist
5.1.1 Bad destination mailbox address
5.1.2 Bad destination system address
5.1.3 Bad destination mailbox address syntax
5.1.4 Destination mailbox address ambiguous
5.1.5 Destination mailbox address valid
5.1.6 Mailbox has moved
5.1.7 Bad sender’s mailbox address syntax
5.1.8 Bad sender’s system address
5.2.0 Other or undefined mailbox status
5.2.1 Mailbox disabled, not accepting messages
5.1.0 Other address status
5.3.0 Other or undefined mail system status
5.4.1 No answer from host
5.4.2 Bad connection
5.4.0 Other or undefined network or routing status
5.4.3 Routing server failure
5.4.4 Unable to route
5.4.7 Delivery time expired
5.5.0 Other or undefined protocol status

The mailbox is full

5.2.2 Mailbox full
5.3.1 Mail system full

Message rejected (for content, size, or similar reasons)

5.2.3 Message length exceeds administrative limit.
5.2.4 Mailing list expansion problem
5.3.4 Message too big for system
5.5.3 Too many recipients
5.7.0 Other or undefined security status
5.7.1 Delivery not authorized, message refused
5.7.2 Mailing list expansion prohibited
5.7.7 Message integrity failure

Various issues with the email server where you sent the email

452 Requested action not taken: insufficient system storage
500 The server could not recognize the command due to a syntax error.
501 A syntax error was encountered in command arguments.
502 This command is not implemented.
503 The server has encountered a bad sequence of commands.
504 A command parameter is not implemented.
552 The action was aborted due to exceeded storage allocation.
554 The transaction failed for some unstated reason.
5.3.2 System not accepting network messages
5.3.3 System not capable of selected features
5.4.5 Network congestion
5.4.6 Routing loop detected
5.5.1 Invalid command
5.5.2 Syntax error
5.5.4 Invalid command arguments
5.5.5 Wrong protocol version
5.6.0 Other or undefined media error
5.6.1 Media not supported
5.6.2 Conversion required and prohibited
5.6.3 Conversion required but not supported
5.6.4 Conversion with loss performed
5.6.5 Conversion failed
5.7.3 Security conversion required but not possible
5.7.4 Security features not supported
5.7.5 Cryptographic failure
5.7.6 Cryptographic algorithm not supported