Thousands of small-business owners have received letters from the Internal Revenue Service questioning whether they are underreporting their business income, a harbinger of a broader initiative aimed at boosting federal tax receipts and ensuring compliance. The program is the latest move in the agency's effort to combat what it sees as a widespread problem: failure by businesses, including mom-and-pops, to report all cash sales in order to minimize tax bills. Tax officials say the letters don't constitute an audit and instead are simply a request for more information. Some business owners and some lawmakers, however, call the new IRS program...