def message_id(data = nil)
      key = 'h:Message-Id'
      return @message.delete(key) if data.to_s.empty?
      set_single(key, data)
    end
def message_id(data = nil)
      key = 'h:Message-Id'
      return @message.delete(key) if data.to_s.empty?
      set_single(key, data)
    end
Set the Message-Id header to a custom value. Don't forget to enclose the
 Message-Id in angle brackets, and ensure the @domain is present. Doesn't
 use simple or complex setters because it should not set value in an array.

 @param [string] data A string of data for the parameter. Passing nil or
   empty string will delete h:Message-Id key and value from @message hash.
 @return [void]