windows live mail will not send emails

I am running windows live mail on windows 8.1. I have 2 email accounts set up on windows live mail. 1 account is a Comcast.net and the other account  is an AT&T.net. I can send and receive emails on the Comcast account, but can only receive on the AT&T account. Both accounts are POP3 protocol. I contacted AT&T and they checked all my settings and said they are correct. I have tried it both with authentication checked and unchecked. It will not send either way. When I check authentication it asks me to verify my user name and password. When I do that it just keeps asking over and over until I closed the pop up window. Then it tells me  the email can't be sent. When I uncheck the authentication box it tells me the email can't be send because it requires authentication.