More than 100,000 AT&T cable modem Internet subscribers in Massachusetts and New Hampshire will have to get new e-mail addresses by March 15.

Customers in the greater Boston area formerly served by MediaOne Group, the cable company bought by AT&T from US West in 1999, with the e-mail ending in mediaone.net will have to change their e-mails to an address ending in attbi.com.

About 630,000 customers nationwide are affected, in areas including Florida, Atlanta, Los Angeles and Minnesota.

The change is the result of a legal challenge to the domain name by an advertising company AT&T inherited when it bought MediaOne.

AT&T spokeswoman Jennifer L. Khoury said customers will receive letters and e-mails advising them of the change and providing a Web page where customers can change their e-mail address and names of personal web pages.

Customers who have been given e-mail and Web site addresses ending in attbroadband.com will also have to change them to attbi.com

AT&T will compensate customers by increasing from four to six the number of e-mail addresses and Web pages customers are allotted as part of their package.

It will also allow a new system allowing customers to check their e-mail using the Web.