From your point of view, the situation is even worse.
MAIL FROM: and
RCPT TO: are only used for the enveloppe addresses, that is the address of the actual sender and the address of the recipients. Those addresses can be controlled by the server, for example either the source or the recipients addresses can be required to be a local valid address.
But what is later shown in the recipient's mailer is the header addresses. So your example could go one step further. Disclaimer: this is just an example - NEVER SEND THAT TO A MAIL ADDRESS THAT IS NOT YOURS
telnet MyEmailServer 25
MAIL FROM: RandomValidEmail@mydomain.com
RCPT TO: AnotherRandomValidEmail@mydomain.com
Subject: Promotion as personal secretary of the US president
... rest of message ...
And AnotherRandomValidAddress will receive a message that looks like coming from what you have put in the
From: header field. Ok some filters could now exist and reject this message of mark it as possibly forged, but you should try it on your favourite smtp server. But once again please only use your own address for the recipient!