Maybe, but there are some cases you need to consider before you make such a change.
1) Does anybody in your company use any kind of external service (for example Survey Monkey, Constant Contact, etc.) to send out emails that appear to be "from" your domain? Even if they aren't doing it today, might they do it in the future?
2) Are there any an outside addresses that forward to your users? For example, assume the gmail account "[email protected]" forwards to "[email protected]", and your user "[email protected]" sends to "[email protected]". In that case, the message will arrive from "outside", but with a "@mycompany.com" From: address.
3) Are any of your users subscribed to external distribution lists that preserve the original "From:" address on messages to the list? For example, if Bob subscribes to "[email protected]" and sends a message, he will receive an inbound message looking something like:
From: [email protected]
To: [email protected]
If your server naively looks at the "From:" header (instead of "Sender:"), it might reject this message because you are receiving it from outside.
Because of all of the above, having a blanket policy of "...from a real sender in our company, the email would never come from outside" is not always feasible.