SMSME - Spoofing of internal addresses

Recently we have received a few email attacks on internal users.

Basically our company email addresses are being spoofed, they are being sent in externally with the same TO and FROM address which relates to an internal users email address.

For example

me@mydomain.com will be in the from and the to address. We are using Microsoft Exchange, is there anything we can do with Symantec Mail Security for Microsoft Exchange to stop it from allowing the senders address to be an internal user if it is coming from an external IP as it will clearly have been spoofed.

We are on version 7.5.0.56

I hope that makes sense!