lib/mail/Mail.php
public function addGlobalHeader($key, $value = null)
    {
        if ($key instanceof Header) {
            $header = $key;
            $this->headers[$header->getKey()]
                = $header->getValue();
            return;
        }
        $this->headers[$key] = (string)$value;
    }
public function addGlobalHeader($key, $value = null)
    {
        if ($key instanceof Header) {
            $header = $key;
            $this->headers[$header->getKey()]
                = $header->getValue();
            return;
        }
        $this->headers[$key] = (string)$value;
    }
public function addGlobalHeader ( $ key , $ value = null ) { if ( $ key instanceof Header ) { $ header = $ key ; $ this -> headers [ $ header -> getKey ( ) ] = $ header -> getValue ( ) ; return ; } $ this -> headers [ $ key ] = ( string ) $ value ; }
Add a header to a Mail object

Note that headers added to Personalization objects override
global headers.

@param string|Header $key Key or Header object
@param string|null $value Value
Add a header to a Mail object