application.rb

module RentARoleModel
  class Application < Rails::Application
    config.action_mailer.default_options = { from: "rent-a-role-model@kaeff.net" }
  end
end

contact_from.rb

def headers
  {
    :subject => "Rent a Role Model request",
    :from => "#{name} <#{Rails.application.config.action_mailer.default_options[:from]}>",
    :to => @user.email,
    :reply_to => email
  }
end

environment_gmail.rb

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :address              => ENV['SMTP_ADDRESS'], # smtp.gmail.com
  :port                 => 587,
  :user_name            => ENV['SMTP_USER'],
  :password             => ENV['SMTP_PASSWORD'],
  :authentication       => 'plain',
  :enable_starttls_auto => true
}

heroku_config.sh

heroku config:add SMTP_ADDRESS=smtp.gmail.com SMTP_USER=rent-a-role-model@gmail.com SMTP_PASSWORD=asdf