Copied from the USPS web site:
Customers be aware of fraudulent package delivery messages sent by email.
Customers may be receiving email messages that allege to be from the U.S. Postal Service that contain fraudulent information about attempted or intercepted package delivery.
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.
The Postal Service Inspection Service is aware of the problem and working hard to resolve the issue and shut down the malicious program.
We regret any inconvenience this may have caused our customers. For additional information on the virus or to report receipt of a message, please contact 1-800-ASK-USPS.