Sherry Brown was trying to do the right thing.

While her son, Army Sgt. Deon Brown, was serving in Iraq, she was managing his bank accounts at home. And when the letter came telling her son had won thousands of dollars, she was happy for him.

"I never played the lottery before, so I didn't know what to expect," she said.

But Ms. Brown had been duped by a scammer and ended up losing about $10,000 for her mistake.

In their biggest campaign to date, the U.S. Postal Service is working to make people aware of scams like the one Ms. Brown faced.

Like many scams, the letter addressed to her son, dated June 24, 2005,...