shawfactor

mattsenate

This plugin is can be used for petitions, events, or any verified list and as far as I know is unique in WordPress.
Creating a petition/list/signup is as easy as adding a shortcode to a post or page (or CPT). From there additional editors are available to configure easch aspect of the sign up. Everything is completely self hosted and all list members become users of your site (not a third parties).
Available shortcodes:
[lh_signing_form] (enable signatories on any post, page, or custom post type)
Unconfirmed signatories: [lh_signing_unconfirmed_count]
Confirmed signatories: [lh_signing_confirmed_count]
Total signatories: [lh_signing_total_count]
[lh_signing_unconfirmed_list] (unformated list)
[lh_signing_confirmed_list] (unformated list)
You can customise the display of the signing form ([lh_signing_form]) via a number of shortcode arguments more information is available <a href="https://lhero.org/portfolio/lh-signing/" title="Additional Plugin documentation" rel="nofollow ugc">here</a>

LH Signing

LH Signing allows you to create configurable validated lists like petitions or event sign ups. It is the only self hosted solution in WP

expertitservicede

Add advanced email address validation using Expert Email Validator API. 
Prevent a site visitor from submitting a form with fake or incorrect email address on your site. 
No more lost leads due to mis-typed email addresses. 
No more wasting your precious time reading and responding to spam leads with incorrect email addresses. 
It hooks into the WordPress core function that is used by most form plugins to validate email address. As a result, it works with most forms.
Learn more about <a href="https://expert-it-service.de/email-validation-tool-en.php" rel="nofollow ugc">Expert Email Validator API</a>.
This plugin requires an API Key which can be obtained for FREE after plugin activation.
Email validation is performed via API on the api.expert-it-service.de.
<h4>Known Compatible Forms</h4>
<ul>
<li>WordPress registration</li>
<li>Contact Form 7</li>
<li>Gravity Forms</li>
<li>Ninja Forms</li>
<li>Jetpack/Grunion contact forms</li>
<li>Any other forms that uses <code>is_email($email)</code> WordPress core function to validate email address field.</li>
<li>Any other forms that use the input type ’email’.</li>
</ul>
<h4>Available Languages</h4>
<ul>
<li>English</li>
</ul>

Expert Email Validator

Does not block the form, qualitatively checks whether the mail can be delivered to the specified address.

Nice plugin

Adds advanced email address validation to forms using Expert Email Validator API. Prevents typos in email address field and eliminates spam submissio …

LH Signing vs Expert Email Validator