I can’t help you out directly, but I had similar errors in this case:
- My user is member of a group
- The group has mail aliases
- Emails sent to the group mail alias are forwarded (also) to my mailbox
- Replying to those emails gave the error
Sender address rejected: not owned by user
There I could solve the issue by adding groups to the allowed senders in main.cf (at smtpd_sender_login_maps=)
Seeing you got this error for new users as well, it may be something is off in LDAP. I struggle with LDAP myself, but there should be a file /etc/postfix/ldap-accounts.cf(or other location, as referred in the smtpd_sender_login_maps), that has an LDAP query returning allowed senders for Postfix.