jamiechong

A lot of registration spam comes from emails with a subdomain. Spammers control a domain such as buymyjunk.com and then attempt to register on your site with emails such as irenemwd@foster2.buymyjunk.com and jakemgm@foster7.buymyjunk.com. This plugin simply blocks all registrations using a subdomain.
Reputable users will have a normal email address from a reasonable domain like @gmail.com, @mylegitbusiness.com, @outlook.com, @myschool.edu, etc.
There is no configuration and no setup. It simply just works. The plugin logs all blocked registrations so you can find potential false-positives. Find a link to this log file on the Plugins page where you activate/deactivate this plugin.

Ban Subdomain Emails

Prevent people from registering with emails that contain a subdomain to help reduce spam.

Brandon Fenning

Since the vast majority of WordPress spam is automated in nature, the goal of BotBlocker is to block comment spam bots and not hinder users. This is accomplished through the use of honeypot technology, which tricks the spambot into identifying itself by performing actions that a human could not.
BotBlocker’s features include:
<ol>
<li>Works out of the box to block spam</li>
<li>Automatically stops spam-bots from commenting</li>
<li>Zero hindrance to users: no CAPTCHAS, math questions, passwords or extra input required</li>
<li>Zero false positives and ignores registered users</li>
<li>Can be configured to completely block spam-bots or flag comments as spam</li>
<li>Spam detection messages can be easily adjusted</li>
<li>No javascript or cookies required</li>
<li>Hooks into <code>wp-comments-post.php</code> to prevent spam-bots from directly submitting spam comments</li>
<li>Should work fine with most customized comment forms</li>
<li>No API keys required</li>
</ol>
http://www.lform.com/botblocker/ for more information.
Plugin by <a href="http://www.lform.com/" rel="nofollow ugc">LFORM, a web design company</a> in New Jersey.

BotBlocker

Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.

Ban Subdomain Emails vs BotBlocker