Require Authentication to Send Emails To Owned Domains

My server is allowing emails to be delivered without authentication, and I am pretty sure this is one of the reasons we are getting lots of emails from our own email addresses. A simple telnet smtp proves this to be the case. $ telnet myserver.com smtp Trying XXX.XXX.XXX.XXX... Connected to myserver.com. Escape character is '^]'. 220-server.myserver.com ESMTP Exim 4.91 #1 Thu, 14 Feb 2019 18:44:58 +0000 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail. HELO myserver.com 250 server.myserver.com Hello yyyyyyyyyyyyyyyy XXX.XXX.XXX.XXX] mail from: john@myserver.com 250 OK rcpt to: john@myserver.com 250 Accepted data d354 Enter message, ending with "." on a line by itself Hello . 250 OK id=1guM0c-0000qF-AP How can this be prevented? Attempting to send emails to other domains like brooke@removed.tld returns 550 Authentication Required, so AFAIK this means my server is not an open relay.