I am using Thunderbird Mail on my windows 10 pc. For years I have had not problems with my mail. Now when I try to send mail with stationary the background for may mail I get this message. 

An error occurred while sending mail.

The mail server responded:  <*** Email address is removed for privacy ***> message rejected. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXSNDR. Please check the message and try again.

I contacted Cox and they stated that for what ever reason the 3rd party mail servers such as Windows Live Mail 2011 & Thunderbird are not communicating properly with Cox. They said the problem is on the 3rd party mail server side.

I also use to create my backgrounds in my Windows Live Mail 2011 on my windows 7 through my Windows live mail, adding stationary and music, I also had the ability to go thru Internet Explorer (which I understand is not longer supported) which was still operable to send pages direct thru Windows Live Mail and has also been stopped for what ever reason. I prefer Windows Live mail to Thunderbird but on windows 10 I am not able to use my Windows Live Mail 2011 with the same capabilities.

Is there some settings that can be adjusted to allow me to continue with the capabilites I recently had to again create my stationary/music pages to send thru Windows Live Mail or even maybe thru Windows Outlook Mail? See the error message I get when I try to send mail from Internet Explorer thru my Windows Live Mail account.

An unknown error has occured.

Subject 'Sweet Peace'

Server error: 550

Server Response: 550 <*** Email address is removed for privacy ***> message rejected. Refer to Error Codes section at https://www.cox.com/resedential/support/email-error-codes,html for more information. AUP#CXSNDR

Server. 'smtp.cox.net'

Windows Live Mail Error ID0x800CCC69

Protocol: SMTP

Port 465

Secure(SSL): Yes