UPS recently reported underwhelming earnings numbers, and things could get worse for its business in the near future.When a stock's yield is more than 7%, it's only natural to question whether it's sustainable. After all, if it were truly safe, it would be a bargain buy, investors would buy it up, and as the price would rise, that yield would come down. When that isn't happening, it's a sign that investors are having second thoughts about the payout and whether it really is worth the risk.Logistics giant Un ...