[dopeelf]

Why not use an applescript in Mail.app. Something like...

If the subject contains:
OF:Projectname:Taskname:Done

Then that task will get ticked off as done.

OF:Inbox:Stuff

Will add a new task to the inbox of OF

OF:Projectname:Newtask

Will add a new task to the project

Anything in the message body could be added as a comment to the task.