I had the same issue after upgrading from 3.4 to 16. The sender policy is not handled directly by postfix, but delegated to Kolab SMTP access policy instead (see postfix master file). The Python code enforcing the actual policy seems to be at /usr/libexec/postfix/kolab_smtp_access_policy, which has indeed changed.
I was able to quickfix the sending issue by replacing the kolab_smtp_access_policy file with the earlier version (from Kolab 3.4). Obviously, this could cause other issues, so give it a try at your own risk I created a bug report as well, perhaps someone with deeper knowledge about the policies could shed some light into this and provide a better solution.