在Yii2中通过Swiftmailer发送电子邮件将无法在外部服务器上运行

I am using Swiftmailer in Yii2 advanced and everything, this includes sending emails, works fine on my xampp server. But when I try it on my external Server (1&1 btw) I get a

Swift_TransportException Connection could not be established with host smtp.1und1.de [Connection timed out #110]

I already googled about it for hours an tried almost every solution including using gmail instead or using ports 465 and 25 with ssl and tls. Also using IP-Adress instead of hostname.

I feel like there could be a firewall or some email options I miss but I have tried everything I can think of.

My code looks like this:

'mailer' => [
            'class' => 'yii\swiftmailer\Mailer',
            'viewPath' => '@common/mail',
            'useFileTransport' => false,
            'transport' => [
                'class' => 'Swift_SmtpTransport',

                'host' => 'smtp.1und1.de',
                'username' => 'mymail@myadress.de',
                'password' => 'mypassword',
                'port' => '587',
                'encryption' => 'tls',
            ],
        ],

and again, works perfectly fine on localhost. Maybe someone has an idea what I am doing wrong?