• Attachments can be added while rendering the mail template.

  Fixes #16974.

  Christian Felder

• Added #deliver_later, #deliver_now and deprecate #deliver in favour of
  #deliver_now. #deliver_later will enqueue a job to render and deliver
  the mail instead of delivering it right at that moment. The job is enqueued
  using the new Active Job framework in Rails, and will use whatever queue is
  configured for Rails.

  DHH/Abdelkader Boudih/Cristian Bica

• Make ActionMailer::Previews methods class methods. Previously they were
  instance methods and ActionMailer tries to render a message when they
  are called.

  Cristian Bica

• Deprecate *_path helpers in email views. When used they generate
  non-working links and are not the intention of most developers. Instead
  we recommend to use *_url helper.

  Richard Schneeman

• Raise an exception when attachments are added after mail was called.
  This is a safeguard to prevent invalid emails.

  Fixes #16163.

  Yves Senn

• Add config.action_mailer.show_previews configuration option.

  This config option can be used to enable the mail preview in environments
  other than development (such as staging).

  Defaults to true in development and false elsewhere.

  Leonard Garvey

• Allow preview interceptors to be registered through
  config.action_mailer.preview_interceptors.

  See #15739.

  Yves Senn

Please check 4-1-stable for previous changes.