In the latest version I changed this setting to true where it used to be false by default:
<add key="DisableDotNetOpenMail" value="true" />
So previously by default we were using the DotNetOpenMail library to send mail and now by default we are just using the built in ASP.NET System.Net.Mail library.
You might experiment with that setting and see if it helps. Generally it should work the same with either setting though. The reason we changed back to use the normal .NET version is because of issues sending mail in other languages, the .NET version seems to do better at getting the encoding right.
Make sure the default from address in site settings is the same address as the smtp settings.
Hope it helps,
Joe