Yahoo! Inc. issued an apology Thursday over the blocking of e-mails that contained or mentioned the website of Wall Street protesters saying it was unintentional.

The Yahoo! Mail's spam filter prevented e-mails containing the phrase OccupyWallStreet.org from being sent by the political website ThinkProgress. The same e-mails by several YouTube users inviting people to visit OccupyWallStreet.org were blocked.

A message posted on ThinkProgress.com on Tuesday said, "ThinkProgress has reviewed claims that Yahoo is censoring e-mails relating to the protest and found that after several attempts on multiple accounts, we too were prevented from sending messages about the ‘Occupy Wall Street' demonstrations."

ThinkProgress tried e-mailing other protest websites, like AmericansforProsperity.org and TeaPartyPatriots.org, and the e-mails were sent.

Bloomberg cited Yahoo! as saying in its e-mailed apology that "the domain ‘occupywallst.org' was being caught by one of our spam filters when some users tried to send messages containing it."

The Internet company said, "This was a false positive which we corrected on Monday. However, there was a residual delay (up to 24 hours) for users trying to send e-mails with that phrase."

It was unclear why the OccupyWallStreet.org was singled out by the Yahoo! spam filter. Those who sent e-mails containing the phrase received an automatically generated reply that reads: "Your message was not sent. Suspicious activity has been detected on your account. To protect your account and our users, your message has not been sent. If this error continues, please contact Yahoo! Customer Care for further help. We apologize for the inconvenience."

ThinkProgress said it was able to send e-mails containing the OccupyWallStreet.org to some Yahoo! accounts but not to other accounts.

OccupyWallStreet.org called for a demonstration at Wall Street to demand White House's action on "the influence money has over our representatives in Washington." The protest started Saturday and continues to this day.

New York City police have arrested some protesters for wearing masks, disorderly conduct, impeding pedestrian traffic and for refusing to disperse.