The SendFeed Plugin for WordPress allows you to send your latest post from your RSS feed to an external Mailing List Manager in both text and HTML formats. We use Lyris here at TruthMedia but it should work for Majordomo and other list managers that you can post via email to.
It is capable of sending messages out immediately, at predefined intervals such as daily/weekly/monthly or manually. Also, the text and HTML templates are completely customizable on a per feed basis so you can tailor the emails to suit the list or feed you are using.
We have just released the very first edition of the plugin on WordPress.org. You can find it at: