[{"name":"stuv","email":"xyz@gmail.in","phone":"12345678990"},{"name":"pqr","mail":"abc@gmail.com","phone":"9876543210"},{"name":"xyz","nail":"xyz@gmail.in","phone":"2345678901"},{"name":"abc","email":"abc@gmail.com","phone":"4567890123"}]

db.collection.aggregate([{$project:{mailData:{$objectToArray:"$$ROOT"},data:"$$ROOT"}},{$set:{mailData:{$first:{$reduce:{input:"$mailData",initialValue:[],in:{$concatArrays:["$$value",{$cond:[{$regexMatch:{input:{$toString:"$$this.v"},regex:"@"}},["$$this.v"],[]]}]}}}}}},{$group:{_id:"$mailData",dups:{$push:"$data"},count:{"$sum":1}}}])