Mailchimp with Rules
I'm not 100% sure how this works with your setup and Mailchimp, But i have all of my site emails sending through SendGrid using the SMTP Authentication Support module. So far none have been sent to spam. 

One thing to note about SendGrid: Their free account sends your emails through a shared IP. Sometimes they can get blacklisted from time to time. But you can eventually purchase a private ip through them to ensure even better spam protection.