The /var/log/mail.err kept on repeating of missing forwardings and transports files so I checked and they weren't there somehow
Aug 8 08:23:35 mail postfix/proxymap: fatal: open /etc/postfix/forwardings.cf: No such file or directory
Aug 8 08:24:36 mail postfix/proxymap: fatal: open /etc/postfix/transports.cf: No such file or directory
Recreated them, chmod'ed them and chgrp'ed them to postfix group as instructed in falco's howto.
After restarting the server smtp is now working fine once again
Words are not enough to thank you for pointing me to the right direction!!!
P.S. what could have deleted those files in the first place? The server was running fine for 2 months now without a problem.