Hi Mike,
If you have this set to false, I would try changing it to true.
<add key="SMTPRequiresAuthentication" value="false" />
I'm using Yahoo business email account which I got when registering my domain through Yahoo Domains. My settings are like this and they work for me, but of course it varies with different email providers.
<add key="SMTPServer" value="smtp.bizmail.yahoo.com" />
<add key="SMTPRequiresAuthentication" value="true" />
<add key="SMTPPort" value="587" />
<add key="SMTPUser" value="***@****" />
<add key="SMTPPassword" value="***" />
<add key="SMTPUseSsl" value="false" />
obviously I've masked out my user name and password
Hope it helps,
Joe