错误信息：

SSL报错信息

Connection could not be established with host ... [ #0]

TLS报错信息

stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

解决方法

由于Yii2使用swiftmailer较低所以，需要先更新版本。

composer require swiftmailer/swiftmailer

添加streamOptions配置

        'mailer' => [
            // ...
            'transport' => [
                'class' => 'Swift_SmtpTransport',
                // ...
                // 添加配置
                'streamOptions' => [
                    'ssl' => [
                        'allow_self_signed' => true,
                        'verify_peer' => false,
                        'verify_peer_name' => false,
                    ],
                ],
                // ...
            ]
            // ...
        ],