Visitor which takes time to leave a comment usually are really interested in your content and ready to subscribe to your blog.
The Subscribe on Comment Extension adds a checkbox on standard WordPress comment form letting the visitor to subscribe your newsletter while leaving a comment. Of course you should set the email on comments as a required field.
Configuration
You can choose if the subscription checkbox should be already checked or not. Setting it as already checked could lead to unwanted subscriptions, so think twice before setting this option. It strictly depends on the kind of readers you have.
The opt-in mode (single or double) can be forced or left as set in the general subscription configuration. If you blog works with single opt-in mode you should consider to force it to double opt-in mode, specially if you experience a lot of fake email used to comment (even for real comments and not spam).
The welcome email can be disabled. If you already have a plugin which sends notification when a visitor leave a comment, you can avoid to send too many email disabling the welcome message. Note that, the welcome email can be disabled only if the single opt-in mode is used. I double opt-in mode the full confirmation and welcome process is followed.
Lists are optional, but you can set zero, one o more lists to be associated to the new subscriber. It can be an option to classify highly interactive users and target them with special emails.
Questions and Answers
What about if the email is already a subscriber?
Nothing, the subscription process is skipped if the email is already registered in your contact database. The profile is not update as well (name, lists and so on).
Can I set a special list to who leave a comment in a specific page?
Not this is not possible with the current version.
What about spam comments?
The subscription is processed after the spam check by WordPress and Akismet (you have Akismet active, right?). If the comment is set as spam, it is ignored.
If the comment is changed from spam to not spam?
The eventual subscription request during the original comment submission is not retained in the current version.
If the comment is deleted?
The subscription, once stored, is no more connected to the “life” of the comment, hence the subscription is kept as is.
The subscribe checkbox does not appear.
The checkbox is injected using the standard WordPress “hooks” of the standard comment form. Ask your theme developer if the comment form is compatible with the WordPress “comment_form_submit_field” hook. WordPress 4.0 or greater is required.
The checkbox is there but no subscription is stored.
First, check with an email address you’re sure is not already in your database. Second, ask the theme developer if they implemented a custom comment system or they are using the default one of WordPress. The “comment_post” event must be fired, as done by WordPress.
I cannot add the privacy checkbox.
The privacy checkbox is not supported on comment forms. You can solve the problem using as subscription label something like “I ant to subscribe and I accept the privacy policy of this site”.
You can ass a link to the label:
...and I accept the <a href="/privacy" target="_blank">provacy policy</a> of this site.
Extra field, public lists, …
Those fields are not supported in the comment form. We suggest to send a welcome email with the {profile_url} inviting the subscriber to complete the profile and choose the options that best suite her needs.
Redirect to another page after the comment is submitted
The subscription does not change the normal comment submission flow, hence WordPress reloads the comment page possibly showing service messages, like “the comment is waiting moderation”. More themes/plugins with AJAX submission wouldn’t be compatible with a forced redirection. We’re anyway checking this opportunity.