Hi,
I'm not sure if this will make a difference, but one thing you can try is change this to true in Web.config/user.config
<add key="DisableDotNetOpenMail" value="false" />
By default we are using the DotNetOpenMail library, but this switch can make it use the built in .NET smtp classes.
It may produce the same result, but its worth a try.
Hope it helps,
Joe