MySQL: bind-address 0.0.0.0 in my.cnf does not work?
The configuration option bind-address tells MySQL what interfaces to listen on.  By receiving the error Access denied for user 'root'@'mguru.lnx.gr' we can see that is actually working fine.

What you will need to do is modify MySQL's privilege system so that you are able to connect from 'mguru.lnx.gr'.  The potentially confusing part of privileges in MySQL is that it is the combination of a username + the host.

This is covered in the MySQL manual here.