Version 4.0 of Mailchimp for WordPress does not come with many visible changes, but it is undoubtedly the most important update since we've started working on the plugin 4 years ago.
This update upgrades you from the older Mailchimp API (a server the plugin "talks" to) to their most recent API version. Mailchimp's older API will no longer be operational at the end of 2016, so we are forced to follow along.
Don't worry though, this change will allow for a lot of great stuff later on.
What changes for you
Mailchimp made a lot of changes, even completely removing certain functionality.
This means we had to make some changes as well, although we really tried to keep things to a minimum. Make sure to read the following list of changes to ensure things will keep on working as expected on your site.
Send Welcome Email
Mailchimp removed the "send welcome e-mail" option from their API. This feature is now handled from within your list settings in Mailchimp only.
You can find this setting by logging in to your Mailchimp account and then going to Lists > Your List > Settings > List Name & Defaults.
Please note that the final welcome e-mail is only sent if you have enabled double opt-in.
In the new Mailchimp, groupings are now called "interests". This brings along some related (technical) changes, including a new and hopefully easier data format.
If you are using grouping fields in your form, we highly recommend re-adding them to your form code through the "available fields" buttons at the top of your form editor.
Some of the available filter hooks in the plugin were renamed or deprecated. While backwards compatibility is taken care of, we recommend you update your custom code to the new hook names.
Our premium eCommerce360 integration will keep on using the older API versions for now. We are releasing a new e-commerce plugin a few weeks from now. All existing customers with a valid license key will automatically receive this new plugin, free of charge.
Questions? We're here to help
Should you have any questions about the update to version 4.0 or the plugin in general, don't hesitate to let us know. We are here to help and guide you trough the update process.
PS. As usual, you can download the update from WordPress.org. If you're not seeing an available update in your WordPress Admin, please deactivate the plugin and click the "Check Again" button on the updates page.