Rails 4.2.1 (March 19, 2014)

• No changes *

Rails 4.2.0 (December 20, 2014)

• MailerGenerator now generates layouts by default. The HTML mailer layout
  now includes and tags which improve the spam rating in
  some spam detection engines. Mailers now inherit from ApplicationMailer
  which sets the default layout.

  Andy Jeffries

• link_to and url_for now generate URLs by default in templates.
  Passing only_path: false is no longer needed.

  Fixes #16497 and #16589.

  Xavier Noria, Richard Schneeman

• Attachments can now be added while rendering the mail template.

  Fixes #16974.

  Christian Felder

• Add #deliver_later and #deliver_now methods and deprecate #deliver in
  favor of #deliver_now. #deliver_later will enqueue a job to render and
  deliver the mail instead of delivering it immediately. The job is enqueued
  using the new Active Job framework in Rails and will use the queue that you
  have configured in Rails.

  DHH, Abdelkader Boudih, Cristian Bica

• ActionMailer::Previews are now class methods instead of instance methods.

  Cristian Bica

• Deprecate *_path helpers in email views. They generated broken links in
  email views and were not the intention of most developers. The *_url
  helper is recommended instead.

  Richard Schneeman

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

  Fixes #16163.

  Yves Senn

• Add config.action_mailer.show_previews configuration option.

  This configuration 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.