Major companies warned customers about possible hacking attempts over the weekend after millions of consumer email addresses were stolen from a marketing-email service, according to the Associated Press.

Email addresses or client names were the only information stolen in the security breach Friday, according to a press release from Epsilon.

This information could be used to conduct more efficient “phishing” attacks, in which a hacker would send emails claiming to be a company and ask for a consumer’s login information, but instead steal their information to log into their account.

Data was stolen from Kroger Co., TiVo Inc., US Bancorp, JPMorgan Chase & Co., Capital One Financial Corp., Citigroup Inc., Home Shopping Network, Ameriprise, LL Bean Visa Card, McKinsey & Company, Ritz-Carlton Rewards, Marriott Rewards, New York & Co., Brookstone, Walgreen Co., The College Board, Disney Destinations and Best Buy Co. Inc., according to Security Week.

Epsilon has more than 2,500 clients and sends more than 40 billion emails each year.

– By Jill Jensen