I thought the actual conviction on Capone was Mail Fraud.
Of course the fraudulent use of the US Postal system was the tie to tax evasion.
In 1931, Capone was indicted for income tax evasion for the years 1925-29. He was also charged with the misdemeanor of failing to file tax returns for the years 1928 and 1929. The government charged that Capone owed $215,080.48 in taxes from his gambling profits. A third indictment was added, charging Capone with conspiracy to violate Prohibition laws from 1922-31. Capone pleaded guilty to all three charges in the belief that he would be able to plea bargain. However, the judge who presided over the case, Judge James H. Wilkerson, would not make any deals.
http://www.chicagohs.org/history/capone/cpn3a.html