src/Mail.php
private function buildHeaders($required_headers = []) {
    $build_headers = array_merge($this->headers, $required_headers);
    $headers = [];
    foreach ($build_headers as $name => $value) {
      $headers[] = "{$name}: {$value}";
    }
    return implode("\r\n", $headers)."\nThis is a multi-part message in MIME format.\n";
  }
private function buildHeaders($required_headers = []) {
    $build_headers = array_merge($this->headers, $required_headers);
    $headers = [];
    foreach ($build_headers as $name => $value) {
      $headers[] = "{$name}: {$value}";
    }
    return implode("\r\n", $headers)."\nThis is a multi-part message in MIME format.\n";
  }
Create message headers.

@access private
@param array $required_headers Any headers that should append/replace the defined headers.
@return string The headers.