Rails mail interceptor

mail.rb

# config/initializers/mail.rb
ActionMailer::Base.register_interceptor(MailInterceptor) if Rails.env.staging?

mail_interceptor.rb

# app/models/mail_interceptor.rb
class MailInterceptor
  def self.delivering_email(message)
    message.subject = "[#{message.to}] #{message.subject}"
    message.to = "me@example.org"
  end
end