Reduce Spoofed Messages

We are in a Office365-Exchange 2013 Hybrid environment today, slowing migrating users to the cloud.

We seem to receive a lot of spoofed emails and was looking for some tips/tricks to get a better hold of this. Usually these are emails that are sent to internal folks from senior publicly noticed figures such as the CEO, CFO etc.

The emails look legit, even have the correct alias@contoso.com addresses displayed unless you actually dig into the headers to see different. 

All our email is scanned by Microsoft EOP as they are our MX record holders then passed thru to our On-Premises mail users. 

Is it possible to tighten security by setting the on-premises and MSOL servers to never accept or block email sent from the outside when being sent from alias1@contoso.comto alias2@contoso.com?

Thanks in advance.