MailChimp is one the most popular, solid and full-featured email marketing service. It provides a basic free plan so you can start to use it with our plugin without a fee and switch to an advanced plan when your lists are growing up.
You can subscribe users to the selected lists, use form fields to populated Mail Chimp custom fields, enable / disable double opt-in, show confirmation checkboxes into a form, use segmentation.
Another amazing email marketing platform with fair prices, free subscription and outstanding features. You can find here a comparison with other solutions.
Use data from contact forms to subscribe users to groups, populate custom fields, subscribe to different lists depends on the selected confirmation checkboxes.
Zapier supports integration with 500+ services. Connect your forms with ActiveCampaign, AWeber, Campaign Monitor, ConstantContact, ConvertKit, Drip, InfusionSoft, GetResponse, Mad Mimi, Salesforce, SendinBlue. Can even connect your data with Google Sheets, Slack and etc.