I removed freepbx, despite this I see that there is still something else left in the system configuration. Here is for example what I see in the logs:
5.1.1 asterisk@neth7.internal2.lan User doesn’t exist: asterisk@neth7.internal2.lan (in reply to RCPT TO command))
Jun 22 12:40:02 neth7 dovecot: lmtp(16965): Disconnect from local: Successful quit
Jun 22 12:40:02 neth7 postfix/cleanup[16417]: 2F3F818081D33: message-id=20210622104002.2F3F818081D33@neth7.internal2.lan
Jun 22 12:40:02 neth7 postfix/qmgr[1557]: 2F3F818081D33: from=<>, size=3567, nrcpt=1 (queue active)
Jun 22 12:40:02 neth7 postfix/bounce[16583]: 2473318081D2F: sender non-delivery notification: 2F3F818081D33