Thank you for the additional information. It looks like what's in-question here isn't the encryption of the email contents, but rather the transaction itself. It's possible your data center is filtering outgoing SMTP traffic. Please run the following commands from the cPanel server and let us know the output:
telnet gmail-smtp-in.l.google.com 465 telnet gmail-smtp-in.l.google.com 25
It is not working for me.
I'm on CentOS 7.5 (Core).