Hacked via PHPMailer. How to protect yourself?

Question

PHPMailer

Good afternoon!
There is a land. Sending mail uses the PHPMAiler library. Latest version from github.
All fields are processed by the function example Yesterday, he discovered that all files on the host were damaged by a virus - code insertions appeared. Tell me how to protect yourself? It seems that such a vulnerability was fixed in versions older than 5.22
$email = htmlspecialchars($_POST["email"]);

Answer 1

All fields are processed by the example function

and it does nothing to protect you.
How did you find out what it was through phpmailer? Is there another code on the site?
what is it?