Email Service — Notes

Functional

• Receive (SMTP), store, deliver.
• Send via SMTP outbound.
• Per-user folders/labels; threads.
• Full-text search.
• Spam, malware, phishing detection.
• IMAP/POP for clients; web UI.
• Attachments, signatures (S/MIME, PGP optional).

Non-functional

• 99.99% availability for inbound (loss = lost mail).
• Deliver within seconds normally; minutes during incidents.
• 1B+ mailboxes, 10s of B msgs/day.

Capacity

• 50B msgs/day × 50 KB avg = 2.5 PB/day raw.
• Per user: avg 5–10 GB mailbox.
• Attachment dedup saves 30–50%.

Schema (per-user mailbox)

• messages(user_id, msg_id, thread_id, headers, body_ref, labels[], read)
• threads(user_id, thread_id, subject, last_msg_ts)
• index_terms(user_id, term, msg_ids[]) posting list

Trade-offs

• Per-user partitioning simplifies isolation and search but breaks aggregate analytics.
• Bigtable / Cassandra preferred over RDB at scale; transactions are per-user.
• Threading by Subject can over-merge unrelated mails; modern Gmail uses Refs.
• Server-side rules vs client filters: server-side scales but harder UX.
• End-to-end encryption (S/MIME, PGP) cripples search and spam — niche.

Refs

• Gmail architecture talks (Bigtable + search), RFC 5321 (SMTP), RFC 5322 (Mail format), DMARC.org docs, "Gmail outage post-mortems."