AOL for a while has blocked all email messages addressed at its subscribers that reference a campaign opposing the provider's Goodmail Certified Email programme.

Emails containing the DearAOL.com web address on Thursday were bounced back to the sender. A test email sent by vnunet.com on Thursday afternoon was returned. The error message linked to a web page that explained that:

"There is at least one URL in your email that is generating substantial complaints from AOL members."

AOL spokesperson Nicholas Graham said in an emailed statement to vnunet.com that the filtering was due to a "glitch" and that is has since been fixed.

AOL's Goodmail programme is designed to provide legitimate bulk email senders guaranteed passage through spam filters for a fee. The provider claims that it will free up resources to fight spam.

The programme has been sharply criticized by not for profit organisations including the Electronic Frontier Foundation (EFF) who claim that it amounts to an email tax that activists will be unable to pay. In a response to the programme they have set up the Dearaol.com website that is asking people to speak out against the programme.

Activism coordinator Danny O'Brien with the EFF argued that AOL marking the email as spam illustrates what is happening to groups all the time, and proves why the proposed spam plan shouldn't be implemented.

"ISPs like AOL commonly make these kinds of arbitrary decisions – silently banning huge swathes of legitimate mail on the flimsiest of reasons – every day, and no one hears about it," O'Brien said.

"AOL's planned CertifiedEmail system would let them profit from this power by offering to charge legitimate mailers to bypass these malfunctioning filters."

The DealAOL.com initiative over the past month has gathered over 600 organisations. More than 350,000 internet users have signed letters to AOL opposing its pay-to-send proposal.