You can just include _time in your aggregation. Say you want to retain _time of the first event for that MessageID, try like this
index=ExchangeIndex sourcetype=MSExchange:2010:Message tracking
| stats min(_time) as _time values(Recipients) as recip by MessageID, Sender
| table _time, MessageID, Sender, recip
You can also use max(_time) to get latest event's timestamp.
... View more