The problem is that as soon as you set this to true:
<add key="EnableSiteSettingsSmtpSettings" value="true" />
it now ignores the smtp settings from config file and looks for them as site settings which you have not populated.
When you set that to true, a new tab appears in the site settings page for entering the smtp settings.
I also recommend you set DisableDotNetOpenMail back to true and just let it use the System.Net.Mail classes. I may get rid of DotNetOpenMail, it initially offered some promise of features not included in the framework (like multi part messages with both an html and plain text body) but over time it has shown that it has issues of its own. Currently there is no benefit to using it.
Also if RunningInMediumTrust = true in Web.config it will not use DotNetOpenMail either.
Hope it helps,