Pop\Mail\Mail::buildHeaders PHP Method

    protected function buildHeaders()
    {
        $headers = null;
        foreach ($this->headers as $key => $value) {
            $headers .= is_array($value) ? $key . ": " . $value[0] . " <" . $value[1] . ">" . $this->message->getEol() : $key . ": " . $value . $this->message->getEol();
        }
        return $headers;
    }