Dovecot Quotas are not recalculated nor is mail rejected if quota is exceeded

Okay, after some more research I found the solution: Problem was inside the postfix configuration, not dovecot!

Postfix had a wrong entry in the mydestination line - so it never used the virtual transport to deliver the messages to Dovecot, but delivered them directly to the local mailbox.

After removing the entry from mydestination in the postfix config, everything is working as expected. The log when mail is received now also looks different -it is now speaking explicitly about Dovecot:

Aug 21 20:32:39 zame postfix/pipe[26958]: 676243723C8: to=<user@domain>, relay=dovecot, delay=0.85, delays=0.5/0.03/0/0.33, dsn=2.0.0, status=sent (delivered via dovecot service)

Note the relay=dovecot.