[exim] Way to Prevent Spoofed Internal Addresses?

Is there a way with exim to prevent spoofed internal addresses?

ie:, someone has a valid internal account, say person1@???, and they
then go into Outlook and change the from address to person2@??? ?

We have this blocked inbound from the net (not allowing anything from the
internal domain to come in), however, people can still spoof from addresses
internally.

Any ideas?