Upgrade your Akismet version to 2.2.7.
About Akismet : Akismet or Automattic Kismet is a spam filtering service. It attempts to filter link spam from blog comments and spam TrackBack pings. The filter works by combining information about spam captured on all participating blogs, and then using those spam rules to block future spam. Akismet is offered by Automattic, the corporation which employs most of the main developers of the Free Software weblog platform WordPress. Akismet was launched on October 25, 2005. Akismet is said to have captured over 10.1 billion spam comments and pings as of March 2009.