Building a robust email backend is deceptively complex. You're not just moving messages from point A to point B, you're managing multiple protocols, handling massive attachments, filtering malicious content, and organizing millions of messages across labels and folders. Get this architecture wrong, and you'll either waste compute resources on spam or frustrate users with legitimate emails stuck in filters. Architecture Overview An email client backend needs to orchestrate several moving parts in harmony. At the core, you have protocol handlers that speak IMAP and SMTP, translating client requests into backend operations. These connect to a message store (typically optimized for fast retrieval and full-text search), an attachment storage system (usually object storage like S3), and a metadata database tracking user preferences, labels, and folder structures. The critical insight here is separation of concerns.…