Resolve WordPress-Mailchimp Email Issues

My opt-in pop-up on WordPress has stopped firing correctly and, even when it does, Mailchimp throws errors and the confirmation emails never reach the subscriber. I need someone to jump in, trace the fault, and get both sides talking again... (Budget: $30 - $250 USD, Jobs: API, DNS, Email Handling, Email Marketing, Mailchimp, PHP, SMTP, Web Development, WordPress, WordPress Plugin)