1. check your /config/environment.rb and make sure u have written at the
bottom line --> require 'action_mailer/ar_mailer'
2. Ensure that you have written it at the bottom line of
/config/environment/development.rb --> :
config.action_mailer.raise_delivery_errors = false
config.action_mailer.delivery_method = :activerecord
3. if still happened, re-generate your mailer -> script/mailer
bla..bla..bla, please make sure the old folder and file are already
renamed to another folder so it will not replace your old files.