The search string you are looking for is "SMTP server require authentication" (or just "<server product name> authentication"). To be clear, what you're establishing isn't really a "Telnet Connection", it's an SMTP connection that you're typing in by hand using telnet. If you tried to send anything that wasn't valid SMTP traffic, the server would ignore it or disconnect. You could also use clients other than
telnet - anything that supports arbitrary TCP connections would work - and the server has no way to know what program is being run as the client anyhow.
Mind you, even after you set your SMTP server to require authentication, you'll still be able to telnet to it. All that telnet is doing is establishing a TCP connection. There's nowhere in a TCP handshake to supply credentials; that comes after the connection is established (see https://en.wikipedia.org/wiki/SMTP_Authentication for more info).
You should also look into enabling TLS (the same Transport Layer Security protocol used in HTTPS) on your server. Even with authentication, sending mail via an insecure connection allows an attacker to potentially read or even modify the mail. Authenticating over an insecure connection potentially allows an attacker to steal your credentials, or at least replay them to authenticate as you at a later time, unless you use a protocol very specifically designed to be secure against that. Therefore, switching to TLS should be done before authentication.