We ran into an issue after upgrading from PHP 5.5 to 5.6. We were no longer able to send email via the awesome WordPress SMTP Email plugin. Turns out that PHP 5.6 introduced some enhancements to its OpenSSL implementation. Stream wrappers now verify peer certificates and hostnames by default. This was causing the form submissions on our site to fail. Clearly there are some issues with our Postfix config and certs. While we sort out those issues, we were able to "solve" our immediate problem, and disabled peer verification in the plugin without editing core files. Thankfully the plugin developer included a filter that would allow us to modify the options array that is passed to PHPMailer.