Mail chimp is a very popular marketing email blast platform consisting of Audience(List Previously) and we can connect our contact form 7 with ease.

Here is the code that needs to be placed on functions.php On below code the 17 is the contact form 7 id in the back end.

Further the API Key and List ID needs to be pulled form Mailchimp Account on below example we also merged couple of form fields to be sent over mailchimp.

add_action("wpcf7_before_send_mail", "wpcf7_do_something_else",90,1);  
function wpcf7_do_something_else($cf7) {
    $wpcf = WPCF7_ContactForm::get_current();
    $submission = WPCF7_Submission::get_instance();
    $posted_data = $submission->get_posted_data(); 
     if($posted_data['_wpcf7'] == 17){
            $apiKey      = get_field('mailchimp_api_key','option');
            $listId      = get_field('audience_id','option');
            $memberId    = md5(strtolower(  $posted_data['your-email'] ));
            $dataCenter  = substr($apiKey,strpos($apiKey,'-')+1);
            $url = 'https://' . $dataCenter . '.api.mailchimp.com/3.0/lists/' . $listId . '/members/' . $memberId;
            $json = json_encode([
            'email_address' => $posted_data['your-email'],
            'status' => 'subscribed',
            'merge_fields' => [
            'FNAME' => $posted_data['first-name'],
            'LNAME' => $posted_data['last-name'],
            'INTERESTED' => $posted_data['interested-in'],
            'MESSAGE' => $posted_data['your-message'],
            ]
            ]);
            $ch = curl_init($url);
            curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);
            curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
            curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
            curl_setopt($ch, CURLOPT_TIMEOUT, 10);
            curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
            curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
            curl_setopt($ch, CURLOPT_POSTFIELDS, $json);
            $result = curl_exec($ch);
            $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
            curl_close($ch);
     }
    return $cf7;
}