This warning has been issued by USPS.com due to the huge amount of fraudulent emails and calls people have been receiving lately. If you go to their page click the Alert link on their main page.
USPS: "Customers be aware of fraudulent package delivery messages sent by email or phone.
Customers may be receiving email messages or phone calls that allege to be from the U.S. Postal Service that contain fraudulent information about attempted or intercepted package delivery.
For emails: If opened, the messages instruct customers to click on a link to find out more about when they can expect delivery of their "package." Simply delete the message without taking any further action.
For phone calls: Please do not provide any personal information and let the caller know you're not interested and hang-up the phone.
The Postal Inspection Service is aware of the problems and are working hard to resolve the issues and shut down the malicious programs.
We regret any inconvenience this may have caused our customers."