To ensure reliable deliverability for Klaviyo and BigCommerce transactional emails, prioritize proper configuration of SPF, DKIM, and DMARC. Begin by setting up SPF and DKIM records in your DNS with values provided by Klaviyo. For SPF, include all authorized sending sources, staying mindful of the 10 DNS lookup limit. DKIM involves key generation and DNS publication. Implement DMARC, starting with a 'p=none' policy to monitor traffic before gradually increasing restriction. Consider using dedicated sending domains and ensure all websites use TLS encryption. Validate your setup using tools like Mail-tester.com. Transactional emails require the same level of authentication as marketing emails, and be aware of how subdomain DMARC policies interact with the root domain.