What types of Mail Delivery Failed errors are there?

There are a great many different Mail Delivery Failed errors. Below are the most common examples:

1. "550 Requested action not taken: mailbox unavailable or not local": This error means that the recipient's email address is incorrect or that the server cannot deliver the message to the recipient's mailbox.

2. "550 No Such User Here": This error means that the recipient's email address does not exist on the receiving server.

3. "554 Delivery error: This user doesn't have a XXXX (for example Yahoo or Google) account": This error means that the recipient's email address is not a valid email address.

4. "550 5.1.1 User unknown": This error means that the recipient's email address is not known on the receiving server.

5. "451 Temporary local problem - please try later": This error means that a temporary problem on the receiving server is preventing the message from being delivered.

6. "421 Service not available, closing transmission channel": This error means that the server cannot send the message at the moment.

7. "550 5.7.1 Relaying denied": This error means that the server is not configured correctly to send the message to the recipient.

8. "554 5.5.1 Error: no valid recipients": This error means that the message has no valid recipients.

9. "553 5.1.2 Invalid recipient": This error means that the recipient's email address is not valid.

10. "550 5.4.5 Daily sending quota exceeded": This error means that the sender has exceeded their daily sending limit.

11. "550 5.7.0 Mail sending denied": This error means that the server is not configured to send the message.

12. "554 5.2.2 Mailbox full": This error means that the recipient's mailbox is full and cannot receive new messages.

13. "450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered": This error means that the recipient's mailbox is receiving messages too quickly and cannot accept new messages at the moment.

14. "451 4.4.2 Timeout - closing connection. Please try again later": This error means that a timeout occurred while sending the message and the connection was closed. Please try again later.

15. "550 5.7.1 SPF check failed": This error means that the SPF check for the message failed and the message cannot be sent.

16. "550 5.7.1 DMARC check failed": This error means that the DMARC check for the message failed and the message cannot be sent.

17. "550 5.7.1 [CS] Spam message rejected": This error means that the message was rejected as spam and cannot be sent.

18. "550 5.7.1 [X] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users, your message has been blocked": This error means that the system has detected an unusually high number of unsolicited emails coming from the sender's IP address. To protect its users, the receiving system has blocked the message.

19. "550 5.7.1 [X] Your message looks like spam": This error means that the message looks like spam and cannot be sent.

20. "550 5.7.1 [Y] Your message was rejected because it contains too many links": This error means that the message was rejected because it contains too many links.

21. "550 5.7.1 [Z] Your message was rejected because it contains a suspicious attachment": This error means that the message was rejected because it contains a suspicious attachment.

22. "550 5.7.1 [AA] Your message was rejected because it contains a virus": This error means that the message was rejected because it contains a virus.

23. "550 5.7.1 [BB] Your message was rejected because it contains blocked content": This error means that the message was rejected because it contains blocked content.

24. "550 5.7.1 [CC] Your message was rejected because it is too large": This error means that the message was rejected because it is too large.

25. "550 5.7.1 [DD] Your message was rejected because it contains a potential phishing attack": This error means that the message was rejected because it contains a possible phishing attack.

26. "550 5.7.1 [EE] Your message was rejected because it contains potentially malicious content": This error means that the message was rejected because it contains potentially malicious content.

27. "550 5.7.1 [FF] Your message was rejected because it violates our terms of service": This error means that the message was rejected because it violates the receiving party's terms of service.

28. "550 5.7.1 [GG] Your message was rejected because it contains prohibited content": This error means that the message was rejected because it contains prohibited content.