OutgoingEventManager::prepareMultipartMixed

private function prepareMultipartMixed(
	array $eventCollection,
	SyncSection $syncSection,
	array $syncEventList = []
): string
{
	$boundary = $this->generateBoundary();
	$this->setContentTypeHeader($boundary);
	$data = implode('', $this->getBatchItemList(
		$eventCollection,
		$syncSection,
		$syncEventList,
		$boundary,
	));
	$data .= "--{$boundary}--" . self::LINE_SEPARATOR;

	return $data;
}