If got this error when sending email to GMail/Yahoo!/etc via secure channel:
Untrusted TLS connection established to gmail-smtp-in.l.google.com[74.125.24.26]:25: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)
Add this line to /etc/postfix/main.cf
smtp_tls_CApath = /etc/ssl/certs
smtpd_tls_CApath = /etc/ssl/certs
and restart Postfix.
