Can't send email from Apple mail in OS 10.11.2

Is 0x800CCC0F the actual error you get? I thought that was a outlook error, not a Apple mail error. If so, email settings are you using?

Also, do you happen to be running Anti-virus on the Mac? That is usually the cause of the 0F error, but usually people don't have anti-virus on their Mac. 

Do you get a popup dialogue box when you try to send.   When this happens to me, i get a box saying "The sender's address was rejected by the server. Select a different outgoing mail server......".   This usually clears up in 5 minutes to 24 hours.  Cox knows about this problem. It is associated with Apple Mail.  I had this happen a lot but it hasn't happened for the last 3 weeks.

My Cox email stopped working with Apple Mail on December 1st, after nearly 5 years of problem-free use.  When I open Apple Mail client while Cox account is enabled, I immediately get a message that Mail cannot connect to the Cox Server.  If I click on Cancel or OK, the message reappears.  So I could sit there for hours upon hours just clicking the error message!  There are no incoming or outgoing mail in my Apple client since December 1st.  My husband encountered the same problem about two weeks later with Outlook (on a MacBook).  This indicates it is not an Apple problem.

We don't have any idea what changed - no change in the router, network, etc.  I am using Yosemite 10.10.5.

My other email accounts (Yahoo, Google and iCloud) work fine - another indication that the problem is not with Mail.

I can use Cox webmail, which proves that there are no account password issues; but it is not an acceptable solution because the webmail is quite slow and very cumbersome for managing large volumes of email.  I have hundreds of folders set up in Apple Mail and now I have no way to put the cox mail into those folders.

I have searched the support documents and verified the settings for incoming and outgoing.  I tried changing from POP to IMAP and the messages persist.

I have tried 3 times to get support from Cox, twice by chat and once by email.  The first person I chatted with said that she would escalate the issue and have a higher level technician call me back within 24 hours - but no one called.  The second person could tell me nothing more than what was in the support documents and seemed too busy to read what I wrote and did not respond to my questions.  I have not received a response to my email inquiry after a full week.

I searched the Internet for solutions - and seems that others have resolved the problem by getting a different mail client (e.g., Thunderbird) - again that means duplicating hundreds of folders.  

I hope someone has a solution.  If not, then I guess I will try to put pressure on Cox by filing a complaint with FCC.