Design an email service

Requirements

• Send and receive email for 1 billion users, store mailboxes, search and thread messages, and filter spam.
• Durable delivery: an accepted message is never lost.

Architecture

Outgoing: Web/app -> API -> outbound queue -> SMTP senders (per destination domain, rate-limited, retries with backoff)
Incoming: MX servers (SMTP) -> spam and malware filtering -> delivery -> mailbox store -> push notifications
Mailbox store: metadata (folders, flags, threads) in a sharded DB keyed by user; bodies and attachments in object storage
Search: per-user inverted index updated asynchronously

Deep dives

Durability. Acknowledge an incoming SMTP transaction only after the message is written to replicated storage. Outbound messages sit in a durable queue and are retried for days according to SMTP rules before bouncing.

Sharding. Shard everything by user id so a mailbox lives on one shard; large attachments are deduplicated by content hash in object storage.

Deliverability. SPF, DKIM and DMARC authenticate outbound mail; per-domain sending rates and warm-up protect IP reputation.

Spam. Layered: connection reputation, authentication results, content and attachment scanning, and per-user feedback.

Follow-up questions

• How do you implement threading? (Group by the References and In-Reply-To headers, falling back to normalized subjects.)
• How do you search a 50 GB mailbox quickly? (A per-user index sharded with the mailbox, built incrementally.)