We have at least two reports that GpgOL will show OpenPGP encrypted mails which are of type text/html as empty when the outlook setting to only view mails as plain text is set. As we genrally recommend using plain text mails only we should try to support this option better.

This means both checking why they might be displayed empty, and additionally to filter out HTML tags / CSS when displaying the mail. As the current code would show the HTML code in the Messageviewer when plain text is set and we do not have a text/plain MIME part.