The email is sent by the file /include/email_addition.php

It uses the $base_path and $base_url variable which are specified in the admin panel's configuration page, so ensure both the BASE_URL and BASE_PATH are set correctly there.

A common mistake is to add "/directory" to the BASE_PATH. If the script is installed in your website's document root, BASE_PATH should be empty. If the script is installed in a sub directory, then BASE_PATH should contain the name of that directory, eg; installed to a directory named "resources" BASE_PATH would be "/resources"

If you simply want to customize the email, you can do so by editing
/include/email_addition.php

You'll notice it uses variables from the language file like $email_addition_4 and $email_addition_4a

These variables are defined in the language file /include/lang/english.php