Mailchimp is our favourite email marketing system.  We use it for our client's sites as well as our own.  It is nice to use, good value, has great features and great deliverability.  We love it!

To help us automate sending form submissions for Silverstripe's user forms module to our Mailchimp lists we have built a module.  The current feature set is already quite comprehensive and the module is under active development.  Feature requests, pull requests and bug reports are welcome! 

https://github.com/asecondwill/mailchimp-subscription-field

The module installs via composer as per best practice with SilverStripe modules.  Once installed a new field type is made available. 

Add the field and save the page.  Now you can click to edit the field.  Choose a list from the ListID dropdown. Click save to record that.  Once the list selection has been saved, the other options will display. 

That's it.  Once you have set the options and published, your user submissions should be sent to Mailchimp.  Happy marketing!