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Justin Sullivan

Canadians are among millions of customers whose names and e-mail addresses have been exposed after a major security breach at a U.S. marketing firm.

Best Buy has notified its customers in the U.S. and Canada that files containing that information have been accessed by an "unauthorized party."

The company in charge of the personal data, a Dallas marketing firm called Epsilon, said last Friday that it was investigating following the discovery of the breach of some customer client data.

Best Buy is warning its customers and Reward Zone members to delete e-mail messages asking for personal information.

The Air Miles Reward Program issued a similar warning Monday after admitting that the names and e-mail addresses of some of their Air Miles collectors have been exposed.

"We have been assured that details of their customers' accounts were not stored in the same system and were not at risk.

Epsilon, a leading marketing services firm, sends more than 40 billion e-mails a year and has more than 2,500 business clients.

The company said that while hackers had stolen customer e-mail addresses, a rigorous assessment determined that no other personal information was compromised.

Experts say that without passwords and other sensitive data, e-mail addresses are of little use to criminals. But they can be used to craft dangerous online attacks.

The information could help criminals send highly personalized e-mails to victims. Doing so makes the e-mail more likely to get past a spam filter.