D'Arcy Norman

A very simple mu-plugin to automatically (automattically?) insert a “Spam prevention powered by Akismet” notice (with hyperlink to Akismet.com) in the footer of each blog hosted on a WordPress Multiuser service to abide by the licensing terms of Akismet. Adds a small “Spam prevention powered by Akismet” to the footer of every blog to abide by Akismet licensing terms.

Akismet Credit Inserter

Adds a small "Spam prevention powered by Akismet" to the footer of every blog to abide by Akismet licensing terms.

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.

Akismet Credit Inserter vs Ban Subdomain Emails