environment.rb

ActionMailer::Base.delivery_method = :smtp
ActionMailer::Base.perform_deliveries = true
ActionMailer::Base.raise_delivery_errors = true

ActionMailer::Base.smtp_settings = {
  :address              => "smtp.gmail.com",
  :port                 => 587,
  :domain               => "gmail.com",
  :user_name            => ENV["SMTP_USERNAME"], # <-- this line
  :password             => ENV["SMTP_PASSWORD"], # <-- this line
  :authentication       => "plain",
  :enable_starttls_auto => true
}

# then run:
#   heroku config:set SMTP_USERNAME=myusername
#   heroku config:set SMTP_PASSWORD=mypassword

script-rails

#! /bin/bash

export SMTP_USERNAME="aenoae"
export SMTP_PASSWORD="oiafeinoaef"

./bin/rails