Mailchimp is a marketing automation platform and email marketing service. Mailchimp is an out-of-the-box integration for Core that enables sending transactional emails.
Mailchimp Transactional Email is a delivery API that can send transactional emails from websites and applications. Mailchimp Transactional is available as an add-on for Standard or higher monthly Marketing plans.
Mailchimp Transactional and Cleeng
Transactional emails are sent to your customers when certain user actions occur (e.g. registering to your service or requesting a password reset). Using Cleeng, you have three email management options depending on the level of customization.
The Cleeng-Mailchimp integration (also referred to as the Advanced option), allows for more customization to your emails and gives you full control over the visual aspect and content. In this case, you need your own Mailchimp account and you are responsible for email handling and management.
Please see the step-by-step guide to the Cleeng-Mailchimp integration.
You may also want to check what emails are sent to your customers.