spoofed local email addresses

[watkykjy]

How do I stop external users from spoofing local email accounts?

Lets say my domain is domain.com and I have local users:
koos@domain.com
piet@domain.com
me@domain.com

On my account me@domain.com I receive email from external parties spoofed as
Unkown@domain.com or koos@domain.com

How do I stop this so that postfix will only allow a user with a local IP address to send mail as a local user?

Thanks.

[Google]