Database
[{recipient:"One",createdAt:ISODate("2014-01-01T08:00:00Z")},{recipient:"One",createdAt:ISODate("2014-02-15T08:00:00Z")},{recipient:"Two",createdAt:ISODate("2014-02-16T12:05:10Z")}]
Query
db.collection.aggregate([{$sort:{recipient:1,createdAt:1}},{$group:{_id:"$recipient",createdAt:{$last:"$createdAt"}}},{$project:{_id:0,recipient:"$_id",createdAt:"$createdAt"}}])