lib/mail/Mail.php
public function addGlobalHeaders($headers)
    {
        if (current($headers) instanceof Header) {
            foreach ($headers as $header) {
                $this->addGlobalHeader($header);
            }
        } else {
            foreach ($headers as $key => $value) {
                $this->addGlobalHeader($key, $value);
            }
        }
    }
public function addGlobalHeaders($headers)
    {
        if (current($headers) instanceof Header) {
            foreach ($headers as $header) {
                $this->addGlobalHeader($header);
            }
        } else {
            foreach ($headers as $key => $value) {
                $this->addGlobalHeader($key, $value);
            }
        }
    }
public function addGlobalHeaders ( $ headers ) { if ( current ( $ headers ) instanceof Header ) { foreach ( $ headers as $ header ) { $ this -> addGlobalHeader ( $ header ) ; } } else { foreach ( $ headers as $ key => $ value ) { $ this -> addGlobalHeader ( $ key , $ value ) ; } } }
Adds multiple headers to a Mail object

Note that headers added to Personalization objects override
global headers.

@param array|Header[] $headers Array of Header objects
or key values
Adds multiple headers to a Mail object