lotabi on "[Plugin: WP Mail SMTP] OpenSSL changes in PHP 5.6.x"

A better solution (avoid patch WP core).

add these lines on $wpms_options inside wp_mail_smtp.php

'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false
        )

The result should be

global $wpms_options; // This is horrible, should be cleaned up at some point
$wpms_options = array (
        'mail_from' => '',
        'mail_from_name' => '',
        'mailer' => 'smtp',
        'mail_set_return_path' => 'false',
        'smtp_host' => 'localhost',
        'smtp_port' => '25',
        'smtp_ssl' => 'none',
        'smtp_auth' => false,
        'smtp_user' => '',
        'smtp_pass' => '',
        'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false
        )
);

HTH
Lota