protected function retrieveMessageEnvelope($identifier)
	{
		$envelope = null;

		// Fetch the requested anvelope if we have any
		if ( isset($this->messageEnvelopeStore[self::MESSAGE_ENVELOPE_STORE_PREFIX . $identifier]) ) {
			$envelope = $this->messageEnvelopeStore[self::MESSAGE_ENVELOPE_STORE_PREFIX . $identifier];
		}

		return $envelope;
	}