Email error messages: Mail delivery failed: returning message to sender

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 message indicates 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 message indicates 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 message indicates that the recipient's email address is not a valid email address.

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

5. "451 Temporary local problem - please try later" - This error message indicates that there is a temporary problem with the receiving server that prevents the message from being delivered.

6. "421 Service not available, closing transmission channel" - This error message indicates that the server cannot send the message at this time.

7. "550 5.7.1 Relaying denied" - This error message indicates 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 message indicates that the message has no valid recipients.

9. "553 5.1.2 Invalid recipient" - This error message indicates that the recipient's email address is not valid.

10. "550 5.4.5 Daily sending quota exceeded" - This error message indicates that the sender has exceeded their daily sending limit.

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

12. "554 5.2.2 Mailbox full" - This error message indicates 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 message indicates 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 message indicates 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 message indicates that the SPF check of the message failed and the message cannot be sent.

16. "550 5.7.1 DMARC check failed" - This error message indicates that the DMARC check of the message failed and the message cannot be sent.

17. "550 5.7.1 [CS] Spam message rejected" - This error message indicates 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 message indicates that the system has detected an unusually high volume of unsolicited email coming from the sender's IP address. To protect our users, the message has been blocked.

19. "550 5.7.1 [X] Your message looks like spam" - This error message indicates 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 message indicates 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 message indicates 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 message indicates 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 message indicates 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 message indicates 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 message indicates 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 message indicates that the message was rejected because it contains potentially harmful content.

27. "550 5.7.1 [FF] Your message was rejected because it violates our terms of service" - This error message indicates that the message was rejected because it violates our terms of use.

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