Send test mail using telnet via smtps (465) port

Connections to smtp (25) start as unencrypted.
Connections to smtps (465) start/negotiate encryption before any SMTP protocol level communication.
You should get "SMTP greeting message" from SMTP server before sending any SMTP commands.

Classic/standard telnet does not support encryption (ssl - Secure Socket Layer).
You may check if your telnet program supports it.

Linux Debian and Ubuntu

Package telnet-ssl replaces standard telnet by telnet program with ssl support. Such telnet+ supports command line like below:

telnet -z ssl smtp.gmail.com 465

[2020-10-25] Debian provides telnet-ssl packages only for oldstable distributions (sid/stretch/jessie).

One on a few alternatives is provided by gnutls-cli program from gnutls-bin Debian package.

gnutls-cli -p 465 smtp.gmail.com