HTML Editor issues and unable to connect to smtp.cox.net issues from Apple MAIL

My first priority would be to use Apple Mail for all my email needs... however after many hours spent with both COX techs and Apple techs I have not been able to resolve sending emails from MAIL! I am able to "send" via COX Classic WebMail. All server settings are correct on my computer! Everything from my computer has been established as correct by Apple techs.

I was able to use MAIL for several years until I upgraded to Sierra OS x 10.12.4! There is an issue that Cox has with adapting to this upgrade that they do not acknowledge and/or are unwilling to deal with! They end up pointing fingers at Apple for the problems even though as the Error window below indicates... is a server issue!

The error message that I always get when trying to send from MAIL is "Cannot send a message using server Cox"  "If you are not using a new email account, visit http://support.cox.com for additional assistance in correcting this issue."

The "Connection Doctor"  when accessed from this window always shows a green light connection for both ingoing and outgoing email! If I click on "Try Later" the window closes but when "later" comes it still remains the same! I have literally tried dozens of times under all conditions with many techs to no avail! Only receive email all of the time and cannot send email all of the time!

I am making a big deal about using MAIL because I have had many frustrating issues, mostly minor, with using COX WebMail! Since I use email actively I need something better than what Cox has to offer. 

My latest EXTREME frustration with Cox has been to try to use HTML Editor for composing emails! After being shunned by a tech who said I needed to go on the internet for a solution to using Cox HTML Editor!!! Cox would not help me with this!!! Why the h--- does Cox have this in WebMail if they cannot support it?

Anyway I found a web site that took me step by step through what I needed to do with Java to get started. I did EVERYTHING as instructed and still could not use the HTML Editor as get error that "Certificate has expired"!

Needless to say I am in a rage about all of this! There supposedly is a solution for every problem... well it is obvious that Cox is not willing to keep up with the technology it subscribes to!