We use Mailchimp for marketing, you probably use Mailchimp too, so we made Mailchimp “Add Contact” Action free. You can now (v0.7.5) export WordPress form submissions to your Mailchimp Audience / List using Tablesome for free.
You can add Mailchimp API key here – WP Dashboard → Tablesome → Tablesome Settings → Integrations → Mailchimp
Do try these out and tell us what you think – https://wordpress.org/support/plugin/tablesome/
WordPress Forms to Mailchimp Tutorial