The package contains two plugins. Post Notification _and_ Post Notification Mailfix. The later is for fixing a bug in WordPress > 2.2. Using WP 2.2 and HTML-Mails you have to activate both of them or use a other Mailingplugin (ie smtp-plugin).
Probably it’s because the Plugin you’re using doesn’t use the Mail – standards. Probably it says “Content-type:” in the code. It must be “Content-Type:”.
