Can’t send messages on Outlook

I run 2 email accounts on Outlook 14.3.6 using OS10.7.5.  One runs perfectly, the other one has just stopped sending messages and I get the message that "the server returned the error 5.7.8 Bad username or password (Authentication failed) Your username or security settings may be incorrect".  I have checked, double checked and redouble checked all the settings and they are correct.  Can anyone advise?