Error 500 when trying to deliver a mail in Quarantine

I have this in the admin-app error log:

[2018-06-12 09:14:37] request.INFO: Matched route "quarantine_deliver". {"route":"quarantine_deliver","route_parameters":{"_controller":"AppBundle\Controller\QuarantineController::deliverAction","date":"20180611","filename":"3BE43C7E-8C9C-4B87-8FBD-B5901951F041.1","_route":"quarantine_deliver"},"request_uri":"https://mail.xxx.de/admin/quarantine/20180611/3BE43C7E-8C9C-4B87-8FBD-B5901951F041.1/deliver","method":"GET"} []

[2018-06-12 09:14:37] security.DEBUG: Read existing security token from the session. {"key":"_security_secured_area","token_class":"Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken"} []

[2018-06-12 09:14:37] security.DEBUG: User was reloaded from a user provider. {"provider":"AppBundle\Entity\BoxRepository","username":"admin@xxx.de"} []

[2018-06-12 09:14:37] request.CRITICAL: Uncaught PHP Exception Symfony\Component\Process\Exception\ProcessFailedException: "The command "'msmtp' '-f' 'Silke-xxx@web.de' 'info@xxx.de'" failed. Exit Code: 75(Unknown error) Working directory: /data/quarantine/20180611 Output: ================ Error Output: ================ msmtp: cannot connect to 127.0.0.1, port 25: Connection refused msmtp: could not send mail (account default from /etc/msmtprc) " at /opt/admin/vendor/symfony/symfony/src/Symfony/Component/Process/Process.php line 239 {"exception":"[object] (Symfony\Component\Process\Exception\ProcessFailedException(code: 0): The command \"'msmtp' '-f' 'Silke-xxx@web.de' 'info@xxx.de'\" failed.\n\nExit Code: 75(Unknown error)\n\nWorking directory: /data/quarantine/20180611\n\nOutput:\n================\n\n\nError Output:\n================\nmsmtp: cannot connect to 127.0.0.1, port 25: Connection refused\nmsmtp: could not send mail (account default from /etc/msmtprc)\n at /opt/admin/vendor/symfony/symfony/src/Symfony/Component/Process/Process.php:239)"} []

[2018-06-12 09:14:37] security.DEBUG: Stored the security token in the session. {"key":"_security_secured_area"} []

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