Php and others...

Version of postfix postconf -d | grep mail_version Version of php php -v Version of mysql mysql -V

for apt-get update error  There is no public key available for the following key IDs  1397DC53640DB751 run this command sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397DC53640DB751 replace  1397DC53640DB751 with your key