SMTP is the protocol by which exim (or any other MTA) receives inbound mail.
If you turn off or block SMTP you don't get email.
Period.
(Unless you outsource your email to another company and let them worry about it.)
And you have to have a way of sending email off the server anyway, so even if you turned off SMTP you'd still want exim, (or sendmail or something) on the server, so the server could notify you if it had a problem.
Jeff