A workaround!
Okay, you can set the ‘Open With’ preference for an email message in the Finder with the good ole Info panel.
- drag an email to the desktop say.
- Cmd+i to get the info panel, choose the Open With popup
- Choose Mailmate (and I presume Spark or Outlook depending on what you have)
- Hit the Change All button and confirm.
And now dragged emails in Agenda, when double-clicked, will launch correctly in your preferred email app.