How to prevent Mail Spoofing

Hello.

I have an Exchange 5.5 server and I created a distribution list “list1”.

The “delivering restrictions” settings of Exchange 5.5 allow me to configure that only myaccount@somewhere.com can send email to list1 and nobody else.

But, the problem is that someone can spoof the email address myaccount@somewhere.com and to succeed in sending email to list1.

Any ideas how can I prevent this from happening?

Thank you in advance.