have you set the SMTP to require a password. That message looks like you have not.

Tools menu (ALt+T) > account settings > outgoing server (SMTP) click edit and make sure a password is required.

Thank you, Matt. It is set to require a password but I still receive the error message. What I find strange is that my email has been working fine for over a year (apparently with the wrong port) and only yesterday did I start receiving this alert.

this is the weird port 80 think that godaddy use. Only mail server in the world probably that deliberately uses the same port as web pages. Confuses anti virus program sometimes.

Please a screen show of the SMTP entry in the account list, so I can see the settings. See How do I create a screenshot of my problem? for instructions if you do not know how.

I am attaching two screen shots. One is the image that I think you are asking for: the SMTP settings. You will see that the port is 465 but I have tried 25, 80 and 3535. The other screen shot shows the error message that I receive. I spent an hour and a half on the phone with someone from Geek Squad and nothing helped. And because my webmail is working fine my email provider says that the problem is not on their side. Thanks so much for your help.

That is exactly what i wanted.... now I see your using a mac... have you installed an anti virus on it?

It was Best Buy who did all installations. They did not tell me that they installed an anti-virus program and I personally have not done so.