Use getOwner instead of getUser

 			} else {

 				$free = $this->storage->free_space($path);

-				$quotaFree = max($this->getQuota() - $used, 0);

+				$ownerId = $this->storage->getUser()->getUID();

+				$ownerId = $this->storage->getOwner($path);

+				$mailQuotaUsage = (int) \OC::$server->getConfig()->getUserValue($ownerId, 'ecloud-accounts', 'mailQuotaUsage', 0);

+				$mailQuotaUsage = max($mailQuotaUsage, 0); // to avoid negative mail quota usage value

+				$quotaFree = max($this->getQuota() - $used - $mailQuotaUsage, 0);