Everybody knows the dangers of putting incriminating information in an e-mail. All it takes is a click on the forward button for your darkest secrets to reach your worst enemies. Beda Singenberger—who lives in Zurich and ran Sinco Treuhand, a wealth management and tax advisory business—discovered that snail mail can do damage, too. Over an 11-year period, federal prosecutors charge, the Swiss financial adviser helped 60 people in the U.S. hide $184 million in secret offshore accounts bearing colorful names such as Real Cool Investments and Wanderlust Foundation. The basis for the charge? According to a prosecutor, Singenberger put a list of his U.S. clients—with incriminating details—in the mail. And now it’s in the hands of federal authorities.