An email address is needed for external sending via Internet.
Since the user is an internal user the mail is just being dropped into his inbox (SBWP transaction).
Similar to something that you put your colleague on his desk, you just walk by and let it drop. No need to prepare an envelope and go to the local post office.
Just try it. Send a a mail from your business workplace to his business workplace. Try it again without internet email address in his user profile