class OsomMailer
helper :some_helper_name
You have to use some_helper_name if your helper is defined as "some_helper_name_helper.rb"
class OsomMailer
helper :some_helper_name
controller.use_rails_error_handling!
in your controllers.
Here you can find an example: http://pastie.org/151743
otherwise the exception will be thrown, and your test will fail :(.