Actionmailer.receive not creating TMail object


I am experimenting with receiving email with actionmailer via getmail
(as described in the Rails Cookbook). The receiving part is working
like a charm but when I check the class of the email object from
inside the receive method I get a string instead of a TMail object, as
I was expecting. Which means I can't do fancy stuff like
email.subject. I have no idea what's wrong. I'm haven't used
Actionmailer much, so I might be just misunderstanding something.

Here's my Actionmailer class:

class Importer < ActionMailer::Base
  def self.receive(email) "Received mail #{email.class}"