how to prevent forged email from exchange?

Hi there, 

we are currently receive alot of email that not belong to us but using same domain, for example, we are using @xyz 

but there is no such users call imnotjack in our organization, and this users keep sending email to my organization,

imnotjack@xyz, accountsdepartment@xyz, sales@xyz. how to prevent this at exchange 2010?

Please advise.

Thanks

Alfred