Mailchimp

Introduction
MailChimp is a marketing automation platform and an email marketing service and a trading name of its operator, Rocket Science Group, an American company founded in 2001 by Ben Chestnut and Dan Kurzius. MailChimp began as a paid service and added a freemium option in 2009.

Getting Started
https://mailchimp.com/resources/mailchimp-101/

Custom DKIM and SPF Authentication
If there are 2 email providers (such as GMail and Mailchimp) to consider for enhanced authentication, it is recommended that clients change their SPF records so that they have a single SPF record including both Google Apps email and Mailchimp SPF requirements in a single record thus...

v=spf1 include:_spf.google.com include:servers.mcsv.net ~all

https://organicweb.com.au/marketing/mailchimp-spf/

https://mailchimp.com/help/about-email-authentication/