I don't remember why smtp_tls_cafile is defaulted to /etc/postfix/cacert.pem on
Ubuntu, but according to the man page of update-ca-certificates
/etc/ssl/certs/ca-certificates.crt is the right file.
http://manpages.ubuntu.com/manpages/precise/man8/update-ca-certificates.8.html
Merge pull request #185 from phunehehe/postfix

Use the right CA file for Postfix