Add pluggable transport registry

django_mailbox/transports/base.py

@@ -17,3 +17,10 @@ class EmailTransport(object):
             message = email.message_from_string(contents)
 
         return message
+
+
+class PluggableEmailTransport(EmailTransport):
+    @classmethod
+    def from_uri(cls, uri):
+        raise NotImplemented("The class does not implement {0}.from_uri, and it should. "
+                             "Overwrite {0}.from_uri.".format(cls.__name__))