MLD email newsletter plugin

[leenm]

I'm using the new version 3.1

That shouldn't be a problem, I'm using it with 3.1 also.
I understand you're using the SMTP authentication settings. Those are usually not necessary when sending email from within your webhost (using the localhost email server). From your local installation: email servers usually don't allow connections from outside, so you can't use your webhost email server for testing from localhost.

Are there any php error messages? (use the debug plugin for quick enable/disable debugmode: http://get-simple.info/extend/plugin/debugmode/130/)