Block all email domain senders except my domain

Question

Is there a way that I can only send emails from my domain? 
 block all other domains so that users can only send emails through the company email?

PhilippRusch · Answer

Just do not allow the typical Mail ports to reach the internet (SMTP-25, Mail delivery 465, 587, maybe others) 
 Then your internal users are forced to use your internal mailserver. 
 The rest of the checks has to be made by the mailserver (who is allowed to send from which domain and so on)