Can no longer send mail from my iMac

I have been able to send & receive emails until today when in the middle of typing an email it appeared that my Internet went down briefly. When I went to send the email I got the message: Cannot send message using the server cox.net. The smtp connection to server "smtp.cox.net" failed. Select a different outgoing mail server from the list below or click try Later. The only option highlighted in the box is " cox.net" (Offline).

I have gone to the Connection Doctor. It says:

"Mail was able to connect to the Internet."

"Trying to log in to this SMTP account failed. Verify that the username & password are correct."

Which I have verified & are correct. 

I am on an iMac OS X Version 10.9.1

Any help would be greatly appreciated!