Fake email account using our domain

Suppose I’ve a domain named abc.com. I m creating email addresses of my own usres, such as a@abc.com, b@abc.com. But, today I’ve received a email from z@abc.com who is not in my list!!!
How it’s possible & how to resolve it?

The name is Job. Joe Job.



Ya… I did read it before, but how to stop this job???

You can’t. Nothing stops people from putting bogus return addresses on email, or postal mail, for that matter.