Sign notification mails with S/MIME

Hi,

I am currently implementing GLPI in my company, and my boss asked me if it was possible to sign with S/MIME the notifications sent by GLPI to the customers, to make sure they come from us.

I saw that GLPI was using PHPMailer : https://github.com/PHPMailer/PHPMailer

and I also saw it supported it.

But I can't seen anywhere in GLPI how to add my signature certificate.