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Akismet takes the pain out of managing comment spam. It ensures that that spam doesn't mess with your search engine rankings, and gives you more time to connect with your readers, instead of laboring through page after page of spam.
Note: This plugin costs $5 a month if you make any money from your site, such as through an affiliate link, ad, or paid service.
The Akismet WordPress plugin ships with WordPress 2.0 and later by default, but if you don't have it for some reason, you can download it here.
After downloading, go to the Plugins page in your administration panel, and click the Activate link in the same row as the Akismet plugin entry. After you've activated it, you'll need to sign up for an Akismet API key (note: API keys should be kept private, like passwords they should never be given out to anyone).
Setting up Askimet is a cinch, once you have your Akismet API key head into your WordPress admin panel. Click the "Akismet Configuration" link under "Plugins" on your left side navigation menu. Finally, enter your Akismet API key into the text box (copying and pasting is a good way), and click on the "Update API Key" button.
Each time a new comment, trackback, or pingback is added to your site it's submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. As a result, you don't have to waste your time sorting through and deleting spammy comments from your blog.
This is also called a "false positive," and is a comment that is not actually spam, but is mistakenly marked as so, since computers aren't perfect. That's okay, it's easy to unmark it and let it back into the stream of legitimate comments.
Go into your administration panel, and click "Comments" from the left menu options, and then click "Spam" at the top of the page. Find the comment, and click on the "Not Spam" button.
Only one comment marked as spam is displayed per IP. The reasoning behind this is that if one comment from that IP is spam, the rest follow suit. You can read more on the support forums about this behavior.
From the comments section of your administration panel, you simply mouse over the spam comment and click the red "spam" link. Marking the comment as spam will submit it to the Akismet servers. The comment will no longer display in the moderation queue or on your site, and it will be automatically deleted by the Akismet plugin after one month. When submitting previously not recognized spam to the Akismet service, Akismet will analyze the content and learn to recognize new spam patterns.