Or another way is to create a Task from the e-mail and convert it into a Project by doing the following (I go this answer from the Plug-in support people):
To convert a task directly into a project go to Tasks, open the task you wish to convert, go to the Project drop down box and clear the Project box, go to the Action drop down box and choose Projects (If Projects is not listed <Add New Action> and type in "Projects". Be sure that it is spelled "Projects".) Then choose Save and Close. This task is then converted to a Project.
Then simply start creating Tasks using the GTD Plug-in with the Project set to the new Project you created from the task (which originally came from the e-mail)