Mailchimp confirmation emails not working

Originally I set up my newsletter sign up boxes to have double opt-in for Squarespace and Mailchimp, but then I realised the confirmation email was coming from Squarespace and not from Mailchimp, and really Squarespace is just a backup for me. So I turned off the Squarespace connection and left the Mailchimp connection and then the Mailchimp connection no longer worked. The double opt-in box is ticked in Squarespace (for Mailchimp) and the double opt-in setting is turned on in Mailchimp. I have tried resetting the Squarespace connection. I have authenticated the domain, and it's still not working? Had any one else had this issue?