EverPress

Verifies your subscribers email addresses. Read more on the <a href="https://kb.mailster.co/reduce-false-signups-with-email-verification/?utm_campaign=wporg&utm_source=Email+Verify+for+Mailster&utm_medium=readme" rel="nofollow ugc">Mailster Knowledge base</a>
This plugin is no longer needed since Mailster 3.0!
<h3>Additional Info</h3>
<blockquote>
This Plugin requires <a href="https://mailster.co/?utm_campaign=wporg&utm_source=Email+Verify+for+Mailster&utm_medium=readme" rel="nofollow ugc">Mailster Newsletter Plugin for WordPress</a>
</blockquote>

Mailster Email Verify

The plugin will reject the email address if there is no such domain, but the SMTP ping to see whether the actual email address exists or not doesn't work.
Without this, it's not really useful.

MX checking works, SMTP doesn't

Verifies your subscribers email addresses. Read more on the Mailster Knowledge base

Xaver

Mailster Newsletter Plugin

Integrates Mailster Newsletter Plugin with Gravity Forms to subscribe users with a Gravity Form. 
Requires the Mailster Newsletter plugin and the Gravity Forms plugin
<blockquote>
This Plugin requires <a href="https://mailster.co/?utm_campaign=wporg&utm_source=wordpress.org&utm_medium=readme&utm_term=Gravity+Forms" rel="nofollow ugc">Mailster Newsletter Plugin for WordPress</a>
</blockquote>

Mailster Gravity Forms

Integrates Mailster Newsletter Plugin with Gravity Forms to subscribe users with a Gravity Form.

Mailster Email Verify vs Mailster Gravity Forms