External sender requires SMG to advertise the SMTP extension "STARTTLS" to proceed in sending an encrypted message. When checking the packet capture, we don't find any SMTP extension advertise.
Sender starting the SMTP session with "HELO" instead of "EHLO"
Sender needs to use "EHLO" instead of "HELO" to take advantage of a secure connection for SMTP and comply with RFC 3207.
For more details please refer to the below article: