When postfix sends email to other server then postfix will act as SMTP client. Therefore the you need to refer to related document about SMTP client and TLS.
To activate TLS encryption feature for postfix SMTP client, you need to put this line in
smtp_tls_security_level = may
It will put postfix SMTP client into Opportunistic-TLS-mode, i.e. SMTP transaction is encrypted if the STARTTLS ESMTP feature is supported by the server. Otherwise, messages are sent in the clear.
To find out whether SMTP transaction is encrypted or not, increase
smtp_tls_loglevel to 1
smtp_tls_loglevel = 1
With this config, postfix will has log line like this SMTP transaction is encrypted.
postfix-2nd/smtp: Trusted TLS connection established to gmail-smtp-in.l.google.com[188.8.131.52]:25: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)
When you're finished editing the config file, then remember to execute:
To make the changes take effect.
Note: Your config above only cover Postfix SMTP server
smtpd, a daemon used to receive email.