AOL cited "tepid support" for Microsoft's so-called Sender ID technology, which seeks to cut down on junk e-mail by making it difficult for spammers to forge e-mail headers and addresses, a common technique for hiding their origins.
Thursday's announcement came on the heels of a recent decision by Internet engineers to reject a preliminary proposal from Microsoft because of its patent claims.
In a statement, AOL spokesman Nicholas Graham said the company's objections had nothing to do with Microsoft's patent, but that "AOL has been especially concerned at the lack of acceptance for Sender ID among the free and open-source online community."
Open-source proponents like the Apache Software Foundation had objected to Microsoft's plans to prohibit software developers from further licensing the Sender ID technology to others even though the initial license would be free.
A working group of the Internet Engineering Task Force, which is in charge with setting technical standards for the Internet, is currently working to develop a way to let software developers decide whether to opt for a non-patented alternative.
Microsoft spokesman Sean Sundwall described AOL's decision to back the alternative as consistent with those efforts.
AOL still plans to submit lists of its mail servers' unique numeric addresses so that other service providers may use the Microsoft technology to verify that messages carrying AOL addresses were actually processed by an AOL server. But for incoming mail, AOL will use a different, nonproprietary method of checking.
By Anick Jesdanun