Postfix default MySQL configuration is not correct (wrong SQL host and relay_domains lacks proxy:)
These two issues are installer related as these configurations are created/modified by it.
ISPConfig creates /etc/postfix/mysql-virtual_*.cf files with : hosts = 127.0.0.1
And the MySQL "dbispconfig" user hostname is "localhost" by default, which does not allow connexions from TCP/IP but only Socket, so you will have an error informing you that Postfix cannot login to the database with an error such as this one : Aug 13 12:47:05 hostname postfix/proxymap[17382]: warning: connect to mysql server 127.0.0.1: Access denied for user 'ispconfig'@'127.0.0.1' (using password: YES)
In order to correct that issue : perl -i -pe 's/hosts = 127.0.0.1$/hosts = localhost\n/' /etc/postfix/mysql-virtual_*.cf /etc/init.d/postfix reload
The other issue is that the relay_domains mysql wrapper requires to be loaded using proxy: but its not done by ISPConfig install, so you will end up with this kind of errors in your syslog : Aug 13 13:03:36 hostname postfix/trivial-rewrite[24143]: warning: connect to mysql server unix:/var/run/mysqld/mysqld.sock: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) Aug 13 13:03:36 hostname postfix/trivial-rewrite[24143]: fatal: mysql:/etc/postfix/mysql-virtual_relaydomains.cf(0,lock|fold_fix): table lookup problem
In order to correct that issue : perl -i -pe 's#^relay_domains = mysql:#relay_domains = proxy:mysql:#' /etc/postfix/main.cf /etc/init.d/postfix reload