Staying Compliant With MailChimp

Hello All,

I have recently gotten into the email marketing world and I've been thrown immediately into the fire. At my job we are just getting started emailng our users. So the first thing I did was import all of our existing registrants to our MailChimp list thinking it was fine since these were people who registered on our website. Unfortunately, not too long after, MailChimp flagged our account and told us to remove the users since they had detected an unacceptable level of bounces. Our site has existed for a few years, so some of these users have been registered for years.

My question is - how can we get thee users back into MailChimp? We are currently mailing newly registered users [who are added to our list via an API when they sign up on our site], which is going well so far, but my bosses really want to start mailing our existing user base. We tried a reconfirmation campaign (which MailChimp recommended) through Intercom (a service we used before I came on board) but we didn't get a great result. Are we better off just not adding these users to our list since?