Mailnotifcations are send with the wrong address

CMK version: 2.0.0p21
OS version: Ubuntu 20.04

Error message:
You can see the notifications trying to be send in mail.log - but they are send using the wrong mail address ion scheme of sitename@localhostname. The the smtp and mailutils is therefore sending an error message that sitname@localhost is not allowed to send any mails by using the configures mail address. When I’m trying to send any mails using the OMD [sitename] manually like echo “” mail -s a.s.o. there is no problem and mails are beeing delivered correctly.
Have you got any ideas how to change this strange behavior.
Kind regards

Inside your notification rules you can define the “sent as”.
If nothing is configured there the system uses the default “sitename@hostname” as sender.
That’s correct.