Martin Lormes

If a comment is held for moderation an email message is sent to the comment author with a link to verify the comment author’s email address. When the comment author clicks on that link the comment gets approved immediately. This makes discussions more lively as users don’t have to wait for the blog admin to approve the comment.
Blog owners may also choose to hold the comments in the moderation queue even after successful verification. The verification status is shown in the comment lists in the admin.
If an author has a previously approved comment and his comments gets approved automatically according to the ‘comment_whitelist’ option in WordPress no email is sent.
If a comment is classified as spam by Akismet or another anti-spam plugin no email is sent.
This plugin uses the <code>commentmeta</code> table and thus requires WordPress 2.9
PHP5 strongly recommended

Comment E-Mail Verification

Wish it supported embedded html in the verification email. One note. You must have Settings -> Discussion -> Comment must be manually approved checked in order for it to work. Once I figured that out, it works just fine.

Still works

If a comment is held for moderation an email message is sent to the comment author with a link to verify the comment author's email address.

Aurovrata Venet

Use this CF7 extension to enable OTP links to be inserted into confirmation/notification emails sent to email addresses submitted through your CF7 forms. The OTP is a unique link the email inbox user needs to click to confirm their address. The link is valid by default for 72 hours and can be customised.

OTP by Email for Contact Form 7

A small Contact Form 7 extension plugin to enable email confirmation by unique links sent to the email inbox.

Comment E-Mail Verification vs OTP by Email for Contact Form 7