Please try your test again on port 25, which is the port that server-to-server e-mail would use, as the post above yours said that it is possible to send e-mail

Please try your test again on port 25, which is the port that server-to-server e-mail would use, as the post above yours said that it is possible to send e-mail to @gmail.com without using STARTTLS. This is, in fact, not only possible but required. Otherwise, @gmail.com would be inaccessible to any e-mail address using an MTA that does not support or is not configured for TLS.