[brazzledazzle]

Should be noted that telnet only works when you have unauthenticated access to your mail server.

[susam]

I remember performing authentication via telnet session as well: https://susam.net/blog/auth-cram-md5.html (2011)

Of course, these days, the connection should be wrapped within a TLS session and openssl s_client could be a good alternative to telnet for doing that.