My apple mail is not sending emails on my computer but works on iphone, ipad.

I cant send any emails but can receive emails. This has been going on for a week. Been on the phone with Cox and tried everything. They say it is an Apple issue. Tried deleting the email accounts and still no resolution. Says "cannot send message using the server smtp.west.cox.net" I tried changing the server, the port and still nothing. Anyone have any ideas? This just happened overnight about a week ago.

Thanks!