Hi Bryan,
The solution to the problem is given to you in the error. The server response was: Authenticate first (#5.5.0). You need to set the following in your web.config or user.config:
<add key="SMTPRequiresAuthentication" value="true" />
<add key="SMTPUser" value="UserName" />
<add key="SMTPPassword" value="UPassword" />
HTH,
Joe D.