The Internet service temporarily stops e-mail coming through servers from Microsoft's bCentral, a Web service focused on small businesses.
An AOL representative said the problem cropped up when bCentral servers did not respond to queries from AOL to determine where the mail was coming from, leading the system to "time out."
As a result, AOL temporarily suspended access to bCentral servers, the representative said. Access was restored by Monday evening, he added.
Microsoft said it was aware of the problem and has been working to resolve it.
"We are working diligently with AOL to get this issue resolved ASAP," a Microsoft representative wrote in an e-mail interview.
bCentral is a Web site aimed at providing small businesses with software and services, such as enabling businesses to share schedules and letting customers to make appointments and reservations over the Web.