You are not able to send to gmail.com (or another individual domain) and get the response, "Email not encrypted in transit"
You are sending email to a domain that requires the use of TLS.
Release : 10.7.x
Make sure the you are attempting TLS connections for all traffic going out to the internet.
Verify "Attempt TLS encryption for delivery of all messages" is enabled in (Administration > Configuration > Select a scanner > Edit > SMTP > Advanced Settings > Delivery)