Google's John Mueller said that when it comes to your HTML head section, you want to ensure it is clean and tight. If possible, you don't want it to have broken syntax because it can break other parts ...

// check if e-mail adresses are valid. if (!clsMail::isemail($this->to)) die('ERROR: Invalid To e-mail adres'); if (!clsMail::isemail($this->from)) die('ERROR ...