After the update to 5.0.1 the bug is still there. None of the attendees gets the mail reminder. An appointment with mail notification of the organizer only isn't working, too.

I forgot to mention that I'm using SOGo on NethServer 7.

Have you enabled SMTP authentication (SOGoSMTPAuthenticationType would be defined in your sogo.conf). If so, make sure to pass a credentials file to sogo-ealarms-notify. Example:

sogo-ealarms-notify -p /etc/sogo/sogo.creds

Thank, you. I'm wondering why SOGo can send invitations and delete notifications but no appointment reminders? Do they use separate authentication parameters? I don't know the credentials of the SOGo SMTP server, defined with 127.0.0.1:10587 in sogo.conf. I've never been asked for setting credentials to the SMTP server. Or does SOGo use the smart host of nethserver for sending mails via SMTP?

The appointment reminders would actually be the alarms associated to the appointments.

You probably don't need to enable SMTP authentication in your setup, therefore you could remove SOGoSMTPAuthenticationType from sogo.conf.

I know what the appointment reminders are and that they have to be set up for the appointments.
I now have deaktivated the option in /etc/sogo/sogo.conf: //SOGoSMTPAuthenticationType = "PLAIN";
The invitations and deletion notifications are still working and now the reminders are working, too. :-)