A data breach at one of the world’s largest providers of marketing-email services may have enabled unauthorized people to access the names and email addresses for customers of major financial-services, retailing and other companies.

While no financial information was compromised, the major concern is that the emails and names could be used for “phishing,” that is, phony emails asking unsuspecting consumers to divulge information such as account numbers, log-ins or Social Security numbers.

As a result, companies were warning customers that they don’t ask for personal information, like credit-card and Social Security numbers, via email.

On April 1, Dallas-based Epsilon, a subsidiary of Alliance Data Systems Corp., said that on March 30, “A subset of Epsilon clients’ customer data 1 8 was 3 8 exposed by an unauthorized entry into Epsilon’s email system.

”The information that was obtained was limited to email addresses and/or customer names only,“ the company said, adding that no other information associated with those names was at risk.” The company said an investigation is under way.

Citigroup Inc., one of the affected companies, said it had been told by Epsilon that the information was limited to names and/or email addresses of some North American credit-card customers “and no account information or other information was compromised.”

J.P. Morgan Chase & Co., Barclays PLC, U.S. Bancorp and Capital One Financial Corp. also alerted their customers to the incident.

“As you can imagine, the email that is going out now isn’t going through Epsilon,” one bank representative said.

Walgreen Co., New York & Co. and Kroger Co., also said they were informed by Epsilon that files related to their customers might have been exposed.

SecurityWeek, which follows the Internet- and enterprise-security industry, said other companies affected included Brookstone, McKinsey & Co., Marriott International Inc., Ritz-Carlton and TiVo Inc.