Hi there, @plush 777.
You can connect your QuickBooks Online to Mailchimp and use your data to segment your audience to create custom messaging to get new customers. However, we can't sync new customers to Mailchimp from QuickBooks.
That said, I'd suggest contacting the Mailchimp Customer Support Team if they allow data to sync with QuickBooks. This way, your contacts have synced and review the changes to your audience.
You can open this article for their contact information and the key benefits of integrating with Mailchimp. Also, I'll be adding this article on how to connect with QBO and Mailchimp. Please note that this is US article, but it still works the same: Connect QuickBooks Online and Mailchimp.
Additionally, you can open this link that will guide in managing your customers in QuickBooks: Add and manage customers in QuickBooks Online.
If there's anything else that I can help you with, let me know in the comments. I'll be happy to help.
