E-Mail-Error when after user registration

[DenisMuc]

Hi,
after a registration the email to a new user doesn't get sent and I have an error in my logs.
[2023-01-09 14:17:35] error-log.ERROR: Uncaught Exception Swift_TransportException: "Expected response code 220 but got an empty response" at /var/www/ud01_89/html/refs/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php line 447 {"exception":"[object] (Swift_TransportException(code: 0): Expected response code 220 but got an empty response at /var/www/ud01_89/html/refs/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:447)"} []
I recently updated the page to Silverstripe 4.12.