2.1.9 received-time Field
The received-time field is optional. If this field is present, it contains the received-time-string field, which contains a string that indicates the date and time, in UTC, that the original message was received. The exact format of the string contained in the received-time-string field is left to the implementation.
The received-time field can begin with either "ReceivedUtc" or "Received".<4> The meaning is the same regardless of which string is used.