Can't send email

Whenever I try to send emails from my COX email account using iPhone, WinPhone, Windows10 PC or Windows Vista PC, I get the following error (was able to send email untill now without any issues).

"Server: 'smtp.east.cox.net', Protocol: SMTP, Server Response: '554 eastrmimpo109.cox.net cox connection refused. No, Server Error: 554, Error Number: 0x800CCC6F"

I can still recieve emails. Tried changing settings, etc. using guidlines from COX website but with no success..