CMGI's entertainment start-up iCast is the most recent company to complain that AOL is holding its proprietary AIM service too close by refusing to allow interoperability with rivals.
AOL in the past has insisted that it has the right to bar unauthorized users from the service, which has so far signed up about 45 million customers.
Although no clear legal requirement for interoperability exists, competitors have clamored for access to the expansive AIM directory.
This afternoon, eight companies, including Microsoft, AltaVista and iCast, requested congressional review of AOL's practices in an effort to establish industry-wide standards regarding instant messaging.
"The goal is to bring attention to the continuing need to universal and open access that would allow consumers to instantly talk with their friends, family members and co-workers," said Tom Pilla, a Microsoft spokesman.
In the letter to Senate Commerce Committee members, the authors said they were not interested in government regulations, nor were they taking a position on the AOL-Time Warner megamerger.
"Our sole concern is with ensuring that all Internet users can enjoy the immense benefits of fully interoperable instant messaging capabilities, and avoid the dangers of a Balkanized system," the letter states.
AOL could not immediately be reached for comment. But in previous press accounts on the issue, a spokeswoman said: "We continue to block anyone who attempts to use the AOL infrastructure in an unauthorized way regardless of whether those infringements involve new products or efforts like spamming, hacking or password stealing."
Industry analysts have noted AOL's dominance in the instant messaging market, saying that interoperability should occur only if AIM users are asking for it.
"Until that happens, others should pay to play," Seamus McAteer, an analyst with Jupiter Communications in New York, has said.
iCast's new instant messenger, the iCaster, is supported by Tribal Voice software. It plays video, MP3s, CDs and other audio such as radio broadcasts and allows people to instantly drag and drop music or video into a friend's instant messaging account.
At its unveiling, the company touted that the new product was compatible with AIM without knowing whether AOL was in fact agreeable to the situation.
Around noon PST today, iCast engineers discovered AOL was not game. About 2,000 users of the new service had been locked out of AIM, according to an iCast representative.
Executives from the Woburn, Mass.-based company said they were making an effort to smooth things over with their counterparts at AOL, hoping for a quick resolution.
The situation does not come as a surprise to other companies, such as Microsoft and AT&T, that also have been blocked by AOL.
Last year, after Microsoft launched its MSN Messenger service, the company engaged in a cat-and-mouse game with AOL before admitting defeat in November.
More recently, Tribal Voice repeatedly tried to change its code to gain access to AIM, but each time it was shut out.