Check REMOTE_ADDR against one or more blacklists: If checked, the distributed blacklists check will be enabled. For this to do anything at all, you need to configure at least one DNS suffix.

Enter DNS suffixes for blacklists here: Enter the DNS suffixes for each RBL you wish to use in this box. For example, at the time of this writing, "list.dsbl.org" is a list of open relays, open proxies, and web sites with insecure formmail scripts. To check to see if the poster's IP is on this RBL, you would enter "list.dsbl.org" (without the quotes) into the text box. If you are going to use more than one RBL, enter the suffixes, one per line.

CAPTCHA test for public posts: If checked, this will use a "CAPTCHA" test, currently implemented as an image containing alphanumeric characters that must be re-entered by the poster. This check will be enforced for public posts, and will occur after the poster has finished previewing and has clicked the button to post. Please read the CAPTCHA documentation for more details.

Message preview tokens: If checked, when a user first submits a post to the system, a cookie will be generated that is unique to the thread. When the user submits a post for the final time, the system will look for this cookie, and if it's not found, set the cookie and bring up the preview screen again. This defeats "spam bots" that submit a post (with preview mode disabled) but don't check for a result, because the query was redirected to the preview screen. Note that enabling this feature breaks the profile preference to skip the preview, and also breaks any customization that may have been put in place within the skin to skip the preview -- in other words, with this feature on, everyone must preview before posting.

Suspicious HTTP_REFERER blocking: If checked, this will activate a check to see if the HTTP_REFERER is "suspicious" and block the post if this is determined to be true. The HTTP_REFERER header indicates the page that the browser was last visiting (supposedly) before accessing the form. Since this is easily forged, some bots insert the URL to their own website in this header, which for some sites will cause that URL to appear in site statistics. A "suspicious" referrer is one that is something other than blank, or a page on your site -- the only way a referre can be thusly "suspicious" is if it is being forged in some way (or if there is a web page outside your domain that's linking directly to your posting form, which shouldn't happen).