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Affected subscribers from Comcast's broadband cable service discovered the blocks as early as last Thursday, and they continued to report difficulties through Monday afternoon. By late that afternoon, AOL had fixed the problem, but was unable to provide information as to its nature.
AOL spokesman Andrew Weinstein declined to comment on other Internet service providers that may have been affected, but said the problem was fixed.
"As part of our constant efforts to reduce spam, we made some technical changes to our filters late last week," Weinstein wrote in an e-mail. "A network error that took place during those changes may have inadvertently blocked some e-mail from legitimate sources. That issue has now been resolved, and we regret any inconvenience this may have caused."
It's unclear how many other ISPs were affected, but one reader using SBC Communications' Pacific Bell dial-up service said AOL's spam filters were bouncing his e-mails. An SBC representative did not immediately return a request for comment. AOL's Weinstein declined to comment on whether pacbell.net e-mail was being affected.
AOL commonly uses server-blocking tactics to thwart spammers. Large Internet service providers such as Comcast must register their e-mail server configurations to communicate with AOL. Often, AOL blocks servers originating from large ISPs if they fail to inform AOL of changes or additions to their e-mail servers.
AOL also blocks individuals who are using their own e-mail servers on a broadband provider's pipes. The company in April began blockingsubscribers who have set up their own e-mail servers.
While the source of the problem remains unknown, many Comcast users are feeling the effects. Because Comcast and AOL are the largest cable modem and dial-up ISPs, respectively, the blocks have caused headaches for small-business owners that need to communicate with clients.
For Terry L. Heaton, president of Donata Communications in Hermitage, Tenn., the block has derailed a Web site development project for a client in New York.
"My client in New York is sitting there and trying to send her links and graphics," Heaton said in an interview. "It's frustrating."