"There should be no cost for particular services, and email should be free and accessible to all. This will disenfranchise people," said Richard Cox, chief information officer of anti-spam organisation Spamhaus.
The companies concerned claim that the moves will help to combat spam, but this claim is refuted by Spamhaus.
Richard Cox said: "It won't reduce spam directly. AOL is already good at managing spam issues, and Yahoo! is getting better. It may make it easier to filter mail, and may provide more resources for spam prevention but it could also mean that people lose emails, and so change provider."