Philipp
Thanks for the hint. I can confirm that now Moodle resends failed notification emails.
A Moodle rel. 4.1 was set to send notifications through the local 'sendmail' (the default), which was blocked by the firewall. Once I pointed the
SMTP server to an external one on a different port (587 instead of the blocked 25) the next cron cycle picked up notification and resent it.