Minneapolis-based Target is emailing people who shopped in the company's stores between November 17 and December 15.
During that time, up to 40 million customers' credit card information was compromised.
Target said it expects all emails to be sent by the end of the weekend.
Meanwhile, Target is ramping up capacity at its call center, which has been overwhelmed with calls about the data breach. And the company says it has quadrupled the capacity of its online REDcard account management site in the last 24 hours.
News of the breach just before the busiest shopping weekend of the holiday season. Target is trying to lure customers to its stores this weekend with a ten percent discount on a single transaction.