WordPress:Akismet
Description
WordPress:Akismet is a spam-fighting service that is different from others such as Spam Karma 2 or Bad Behavior in that it checks the content of the comment anonymously with an online server, to determine whether it is spam or not.
The Akismet development team have created a WordPress:WordPress plugin to interface with the online service for comments, which ships by default with WordPress 2.0 and later.
Installing Akismet for WordPress
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 from the official page.
After downloading, go to your Plugins admin page, and click the Activate link in the same row as the Akismet plugin entry. After you've activated it, it will come up with a box below the main navigation saying that the Akismet plugin is not active, and you need to enter your WordPress.com API key.
Setting Up Akismet
No biggie, just head over to WordPress.com and log in to your account (or sign up for one if you don't have one already). Then head into your admin panel if you aren't there already.
Click on the My Account link in the top right corner of the window, and note the API key in the first paragraph under the heading. Now, go to your other site's Plugins > Akismet Configuration page in your admin panel. Then, enter your WordPress.com API key into the text box (copying and pasting is a good way), and click on the "Update API Key" button.
There you go, now you officially have Akismet up and protecting your blog from spam.
Using the WordPress Plugin
Akismet thought my friend's comment was spam!
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 to the Manage > Akismet Spam page. Find the comment, and click on the Not Spam button, and the comment is spam no longer.
Why can't I see all comments held as spam?
Only one comment marked as spam is displayed by IP. The reasoning behind this is that if one comment from that IP is spam, the rest follow suit.
Read more on the support forums on this behavior.
How do I submit missed spam?
If the comment is in the moderation queue, simply check the Spam radio button below that comment, and then click Moderate Comments ». If a comment was already published, select this comment for editing, then set the Comment Status to Spam and click Edit Comment ». Marking the comment as spam will submit it to the Akismet servers. Also, it 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.