mail relay

i got a mail relay server, let say domain @testing.com

this few days, i kept on receiving unknown mail (email not yet created on my e-mail server such as werew@testing.com, gflkj@testing.com) from unknown mail address..such as weerrerter@hotmail.com, dffgfdr@hotmail.com,ersfsdf@hotmail.com…etc. so my email-relay will reply to fgghdfg@hotmail.com which is an unknown mail (maildaemon)

One day can say more than 10,000 this type of mail that floods my email server.. and i cant track is from who??

How to control or to block this spam mail??