Calendar event attendees are not notified

Nextcloud version: 23.0.8 with Calendar 3.4.2
Operating system and version : Ubuntu 20.04
Apache or nginx version : 2.4.41, with php-fpm 7.4.3
PHP version: 7.4

Steps to replicate it:

  1. Create an event in the calendar
  2. Add an attendee
  3. Wait for cron.php to run, or run it manually.

No email is ever sent to the attendee (although it says “Invitation sent”). The email server is configured properly and other emails are being sent properly (e.g. the welcome email). I actually have access to the mail server’s logs, and there’s nothing at all in there. In Admin > Groupware, all checkboxes are on.

I don’t see anything interesting in Admin > Logging or in the apache and php-fpm logs.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'passwordsalt' => 'XXXXXX',
  'secret' => 'XXXXXX',
  'trusted_domains' => 
  array (
    0 => 'nextcloud.digigov.grnet.gr',
  ),
  'datadirectory' => '/var/www/nextcloud.digigov.grnet.gr/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '23.0.8.1',
  'overwrite.cli.url' => 'https://nextcloud.digigov.grnet.gr/',
  'dbname' => 'nextcloud',
  'dbhost' => '83.212.4.238',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'oc_admin',
  'dbpassword' => 'XXXXXXXXXX',
  'installed' => true,
  'instanceid' => 'XXXXXXXXX',
  'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',
  'default_phone_region' => 'GR',
  'memcache.local' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' => 
  array (
    'host' => 'localhost',
    'port' => 6379,
  ),
  'app_install_overwrite' => 
  array (
    0 => 'jitsi',
    1 => 'apporder',
    2 => 'files_external_gdrive',
  ),
  'maintenance' => false,
  'mail_from_address' => 'noreply',
  'mail_smtpmode' => 'smtp',
  'mail_sendmailmode' => 'smtp',
  'mail_smtphost' => 'localhost',
  'mail_smtpport' => '25',
  'mail_domain' => 'grnet.gr',
  'htaccess.RewriteBase' => '/',
  'loglevel' => 0,
  'dbdriveroptions' => 
  array (
    1007 => '/etc/mysql/certs/client-key.pem',
    1008 => '/etc/mysql/certs/client-cert.pem',
    1009 => '/etc/mysql/certs/ca.pem',
    1014 => false,
  ),
  'theme' => '',
);