You may encounter this error when trying to use the MessageViewer link for an email message if:

The email has not been marked public:
  1. Edit the Email Message
  2. Under Include this message in the message list mark "Yes, show this message in the publicly viewable list"
  3. Click Save

The email doesn't have a Plain Text Version added:
  1. Edit the Email Message
  2. Click Plain Text Content
  3. Click Convert to Plain Text
  4. Click Save