Some Freeserve customers are unable to email AOL subscribers because the latter's systems see the messages as spam.
Emails are being bounced back, usually with a message such as: 'The connection to the server has failed'. Users are then given protocol and other error messages.
Some of Freeserve's outgoing email servers are not recognised by AOL, because their configuration means they could be hijacked by spammers.
Freeserve insisted that the problem was easily solved by changing the user's email settings, and that sometimes the problem arose because people use more than one email account.
AOL is now alerting those who have had emails blocked, an AOL spokesman told vnunet.com.
"We are not the only ISP to block Freeserve's email servers. They are more open to spam and so are not a recognised route for sending email to other ISPs," he said.
"We already have an agreed process in place with Freeserve whereby affected customers contact Freeserve technical support to change their settings."
AOL is adding some extra text to the bounced emails explaining how to alter their settings.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago