The remote openSUSE host is missing a security update.
The value of SMTPD_LISTEN_REMOTE accidentally defaulted to 'yes'. The postfix smtp daemon therefore was reachable over the network by default. This update therefore resets the value to 'no' in /etc/sysconfig/mail. If you intentionally want postfix to listen for remote connection you need to manually set it to 'yes' again. This update also fixes a problem where the relay database was not created and postfix refused to start.