異なるフィールドのMongoid同じ埋め込みドキュメントタイプ

これがベストアンサーです！

class Email
  include Mongoid::Document

  embeds_many :to_recipients, :class_name => "Recipient"
  embeds_many :cc_recipients, :class_name => "Recipient"
  embeds_many :bcc_recipients, :class_name => "Recipient"    
  embeds_one :from, :class_name => "Recipient"

  field :subject, type: String
  field :body_text, type: String
  field :body_html, type: String
end

class Recipient
  include Mongoid::Document
  field :email_address, type: String
  field :name, type: String
  validates :email_address, :presence => true
  embedded_in :emails
end