incorrect user name/password

Two days ago I kept getting an 'incorrect username/password' message. Could not do anything with my cox email on my iPhone or my home computer. Changed my password and everything worked for a day. Now I can't send email from my iPhone. I can receive email on my iPhone and email works fine on the computer. I changed my password again, but it did nothing to help so that I could send email from iPhone. Suggestions as to what's wrong?