Website uses firstname.lastname@example.org as email adress
You need to use a plugin or hardcode to change it
You can use the CB Change Mail Sender Plugin for WordPress to change the sender email adress. Alternatively you can change this in the code directly. If you are no coder I would recommend you to go for the plugin as it is in the WordPress repository and it has good reviews. Thus its secure code. If you add code without knowing the secure syntax then this might be insecure for your website.