Same problem here. Adittionaly, im using the feature of send a mail when i write a new post, to the users who are subscribed to this, of course, but now it sends the mail correctly, and another one with a blank template and no subject.
In the google group of mailpress they say that the Developer version should work, but i havent tested yet. I dont want have my users receiving blank mails.
Any solution? I'll do a test right now... to see what happens.
Edit: It seems to work with the tinymce editor on the New mail page. I havent tested it with the automatic sending of mails already...
You can download the Developer version from here: http://wordpress.org/extend/plugins/mailpress/download/