This problem is related to your mySQL permissions.
"Access denied for user: 'firstname.lastname@example.org' " means that the access was denied for user mail_admin that connects from the host mail.abc.de.
You must change the permissions on your mySQL server that the user mail_admin is allowed to connect from mail.abc.de and not only from localhost. Then reload the mysql server. If that wont help, you can try to allow connections from every host, that means to set host to "%" in the mysql permission tables.