When you publish a post the newsletter widget will send a mail to all registered for newsletters.
I noticed that it send a new mail if you're altering a published post. If I, for instance, correct a spelling mistake, i didn't want it to send a new mail.
So in 'Custom/Extensions/Newletter.cs' I changed
Post.Published += Post_Published;
Post.Publishing += Post_Published;
This wil just send mail when publishing. If I want a new mail to be sent I first unpublish the post and then publish again.