Most email marketing service providers frown upon single optins, but several do offer the option to enable it. MailChimp, one of the popular email service provider, allows you to enable single-optin by using their API. Since we already use the API to connect with MailChimp, several of our customers asked us to add the ability to disable double-optins and enable single-optin for MailChimp.
We didn’t want to add a user-interface for this setting because it is not possible for other integrations. If you’re using MailChimp and want to enable single-optin, then simply paste the following code in your theme’s functions.php file:
add_filter( 'optin_monster_mailchimp_double', '__return_false' );
This will disable the default double-optin, and users will be added to your email list without having to confirm their email address.