Wat moet ik doen om dit op te lossen?? Heb ftp toegang tot dat domein, maar verder weinig verstand van scripts.

We urge you to take the necessary steps to fix this problem and to delete the script that is sending the spam. We also ask you to update the user's password and to update any open source scripts and plugins that is used by this user.
Infected script that is sending spam:
/home/****/domains/******.be/public_html/libraries/vendor/phpmailer/phpmailer/class.phpmailer.php
*Possible solutions:*
Install reCaptcha when class.phpmailer.php is used see "Infected script that is sending spam"