Customers have until August 20 to retrieve existing data.
The closure of the list, which Microsoft acquired as part of its LinkExchange in November 1998, reflects the software giant's move to paid Web services such as HailStorm, part of the Microsoft.Net Web services strategy.
ListBot let consumers and hobbyists create mailing lists that automatically sent email updates to subscribers. A more robust paid service, ListBot Gold, was also available. Email lists typically are used to create information communities or distribute newsletters.
ListBot became part of Microsoft's bCentral site in October 1999 with the folding of LinkExchange into the small-business Web site. But after later setting up List Builder, a more business-oriented email management service, Microsoft started to reevaluate ListBot, a company representative said.
The company decided that managing two e-mail list services, one for consumers and another for small businesses, no longer made sense. "Was this really the way to fulfill the mission bCentral had for small-business services?" the representative asked. "No, not really."
The shutdown affects both free and paid ListBot subscribers, who must either choose a competing service or one of two options offered by Microsoft. Those looking for a free, but less-robust, option can turn to Microsoft's MSN Communities. Subscribers may also sign up for List Builder for US$149, or 40 percent off the US$269 annual subscription fee.
Microsoft estimates the shutdown will affect about 90,000 subscribers.
Those wishing to switch to List Builder will be able to migrate their email lists from ListBot. Subscribers moving to another service must retrieve their data before Microsoft closes the Web site.
Microsoft redesigned bCentral in April, offering new small-business services and Web-based tools.
Some ListBot users saw the move as an inevitable part of the push by Microsoft and other technology companies to make money off the Web.
"It doesn't really make sense to offer free services anymore on the Internet," Ben Silverman, who uses ListBot to distribute his newsletter, DotComScoop, told The Associated Press. "It's about revenue now, and if it's not revenue-generating, there's no value from a business perspective."
The Associated Press contributed to this report.