letter_bomb 
Emails are pretty dangerous - inspect your mailer templates from the safety of your browser.
Install
Add to your Gemfile:
gem "letter_bomb", group: :development
and bundle away.
Usage
Mount the engine in your routes.rb:
if Rails.env.development?
mount LetterBomb::Engine, at: "/letter_bomb"
end
and hit /letter_bomb for a list of previews.
Previews can be defined anywhere within app/mailers suffixed with preview,
i.e. app/mailers/user_mailer_preview.rb.
Preview method names are arbitrary so long as they return a Mail object.
class UserMailerPreview < LetterBomb::Preview
def welcome
WelcomeMailer.for_user(User.last)
end
end
Preview methods are wrapped in an ActiveRecord transaction block that will be rolled back after execution, so it's safe to create your own test data within them.
Contributors
Alternatives
Also check out 37signals mail_view, which much inspiration was taken from. See here for more details on the differences.