It seems strange because it is getting s null reference when trying to get a connection with the smtp server
at System.Net.Mail.SmtpConnection.GetConnection
and this would make me think the smtp settings are incorrect.
But, you say it does work if you set <add key="DisableDotNetOpenMail" value="false" />
which means the settings are correct enough for DotNetOpenMail library to send ok, so they should be the same when using the System.Net classes.
But, also you say with DotNetOpenMail, it is only the preview which works but not sending to the list.
The difference is that the preview sends the preview right away, whereas the list creates a task that runs on a background thread and sends the mail top the list members.
The task gets serialized into the database first and then is deserialized on a background thread. Occasionally we have had issues where the task could not deserialize due to localization issues.
Are you seeing any errors in your mojoPortal log related to serialization or tasks?
However, if it can send a preview with DotNetOpenMail, it should also be able to send a preview with the normal System.Net classes. So it is very mysterious.
Best,
Joe