[{name:"a",email:"aaa@bestemail.com"},{name:"b",email:"bbb@bestemail123.com"},{name:"c",email:"ccc@diffemail.com"}]

db.collection.aggregate([{"$addFields":{"matching":{"$cond":[{"$gt":[{"$indexOfCP":["$email","bestemail"]},0]},true,false]}}}])