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.

WPSForm

RedMoon

A lightweight addon for SimpleForm plugin. No configuration required. Once activated, you will find the new “Akismet Anti-Spam” option in the Settings page within the Spam Protection tab. Ensure that before you enable this option, you have activated Akismet plugin and registered an Akismet API Key for enabling anti-spam protection. You no longer need to worry about getting spam in your contact form, since each new form submission will be filtered for spam using Akismet.
For more detailed information refer to the <a href="https://wordpress.org/plugins/simpleform/" rel="ugc">SimpleForm</a> plugin page.
<h3>Demo</h3>
You don’t know SimpleForm yet? Check out our <a href="" rel="nofollow ugc">Demo</a> and find out how it works.

SimpleForm Akismet

Do you get junk emails through your contact form? This plugin for SimpleForm helps prevent spam submission.

Ban Subdomain Emails vs SimpleForm Akismet