The ease of sending and receiving email makes it an attractive way to run scams like phishing attacks. One telltale mark of a phishing attack is the sender’s address not matching their purported domain; attacks that appear to come from legitimate email addresses are much more likely to fool the victim.
You can protect your organization’s email accounts from being compromised and used in phishing attacks by training your users to identify forged emails and use password managers, which won’t autofill a password on a malicious site. But how do you prevent bad guys from forging email that looks like it comes from inside your organization? You can’t, but you can reduce the chances that other email servers will accept it. In the process, you’ll enhance the deliverability of legitimate email from your domain.