I think this may go down as the most valuable skill I’ve learned my entire career.

$ telnet smtp-relay.myisp.com 25
Trying 10.10.32.1...
Connected to smtp-relay.myisp.com.
Escape character is '^]'.
220 myisp.net ESMTP MAIL Service ready - Wed, 18 Oct 2023 16:44:08 GMT
helo localhost
250 myisp.net Hello [10.98.76.54], pleased to meet you
mail from: me@mydomain.com
250 2.1.0 me@mydomain.com... Sender ok
rcpt to: me@mydomain.com
250 2.1.5 me@mydomain.com... Recipient ok
data
354 Enter mail, end with "." on a line by itself
From: me@mydomain.com
To: me@mydomain.com
Subject: Test

Hello there
.
250 2.0.0 39IGi8N5006729 Message accepted for delivery
QUIT
221 2.0.0 myisp.net closing connection
Connection closed by foreign host.