S1 Corporation Board of Directors Recommends S1 Stockholders Not Tender Their Shares Pursuant to ACI Exchange Offer

S1 Corporation

announced today, pursuant to a letter being mailed to its stockholders, that the S1 Board of Directors has determined that the unsolicited and conditional exchange offer announced on August 30, 2011 by ACI Worldwide, Inc. is not in the best interests of S1 or its stockholders and that the S1 Board of Directors recommends that S1 stockholders reject the Exchange Offer and not tender their shares to ACI pursuant to the Exchange Offer.