Action Mailbox 基类

所有应用程序邮箱的基类。不建议直接继承。请继承自 ApplicationMailbox，因为应用程序特定的路由是在那里配置的。路由通过以下方式指定

class ApplicationMailbox < ActionMailbox::Base
  # Any of the recipients of the mail (whether to, cc, bcc) are matched against the regexp.
  routing /^replies@/i => :replies

  # Any of the recipients of the mail (whether to, cc, bcc) needs to be an exact match for the string.
  routing "help@example.com" => :help

  # Any callable (proc, lambda, etc) object is passed the inbound_email record and is a match if true.
  routing ->(inbound_email) { inbound_email.mail.to.size > 2 } => :multiple_recipients

  # Any object responding to #match? is called with the inbound_email record as an argument. Match if true.
  routing CustomAddress.new => :custom

  # Any inbound_email that has not been already matched will be sent to the BackstopMailbox.
  routing :all => :backstop
end

应用程序邮箱需要覆盖 process 方法，该方法在框架运行回调后被调用。可用的回调包括：before_processing、after_processing 和 around_processing。主要用例是使用 before_processing 回调确保满足处理的某些先决条件。

如果先决条件未满足，可以使用 #bounced! 方法停止处理，该方法将静默地阻止任何进一步的处理，但不会实际发送任何退回通知。您还可以将此行为与调用负责发送实际退回电子邮件的 Action Mailer 类配对。这使用 bounce_with 方法完成，该方法接受 Action Mailer 方法返回的邮件对象，如下所示

class ForwardsMailbox < ApplicationMailbox
  before_processing :ensure_sender_is_a_user

  private
    def ensure_sender_is_a_user
      unless User.exist?(email_address: mail.from)
        bounce_with UserRequiredMailer.missing(inbound_email)
      end
    end
end

在处理入站电子邮件期间，将跟踪状态。在处理开始之前，电子邮件通常具有 pending 状态。处理开始后，在调用回调和 process 方法之前，状态将更改为 processing。如果允许处理完成，状态将更改为 delivered。如果触发退回，则为 bounced。如果未处理的异常冒泡，则为 failed。

可以使用熟悉的 ActiveSupport::Rescuable 方法在类级别处理异常

class ForwardsMailbox < ApplicationMailbox
  rescue_from(ApplicationSpecificVerificationError) { bounced! }
end