Forum Discussion

DrJoe's avatar
New Contributor
4 years ago

Sending Email Errors

Using Thunderbird email with no problems for years. For the past three days every email sent gets this pop-up:

An error occurred while sending mail: The mail server sent an incorrect greeting: cmsmtp blocked. Refer to Error Codes section at for more information. AUP#CXDNS.

Tracking the "error codes" says that there is an issue with the connecting IP address Domain Name System (DNS). To resolve it, I am advised that "the Reverse DNS (rDNS) lookup for your IP address is failing" and that I should "confirm the IP address that sends your email."

The IP address is controlled by Cox... The DNS is controlled by Cox... umm... hello? What's going on at your end that I can't send email?

3 Replies

  • From that error message, that means that the IP address that you are attempting to send from ( does not support reverse DNS which is a requirement for our mail servers. This helps prevent against spammers, hackers, etc. from sending mail using our servers. This is not something that Cox can fix as it comes from the sending IP. That IP address is not a Cox IP.

    Cox Support Forum Moderator
  • Festus500's avatar
    New Contributor

    DrJoe, I am having same issue.  Your post and BrianM's reply below has been very helpful.  Here are some more bread crumbs.  The IP address is given to you from your internet service provider. (That is why my phone email is working and my computer is not)  I checked my IP address and it is NOT blacklisted but COX is blocking it for some unknown reason.  I called Tech Support and endured 45 mins of hold then 45 mins of answering silly Tier 1 pre-canned questions by a person who can't spell DNS.  Then I was forwarded to Tier 2.  They were very helpful and elevated my ticket to Tier 3 - who is supposed to remove the COX block on my IP.   

    BrianM - it would be extremely helpful if each of the error codes on your site came with additional information on how the user could remedy - in this case an email to Tier 3 would save both Cox and Customer Time.  It would also be helpful to have Cox notify users when they decide to block their IP?  At least then we could get to bottom (or root cause) of why it is being blocked.

    WiderMouthOpen - MxToolbox works great!

    • JonathanJ's avatar
      Former Moderator

      Thanks for your comment we will forward it to our engineering team.

      Jonathan J
      Cox Moderator