Design an email system (Gmail)

The problem

An email system receives and sends messages for billions of mailboxes, stores them for years, keeps spam out, and lets users search and organise (labels, threads).

Requirements

• Reliable delivery: a message is never silently lost.
• Interoperability with every other mail provider.
• Spam, phishing and virus filtering.
• Large attachments, fast search, and strong privacy.

How email moves between providers

Email uses SMTP to send between servers. The key ideas:

• MX records in DNS tell you which servers accept mail for a domain.
• Delivery is store-and-forward: a server accepts a message, persists it, and passes it on, retrying if the next hop is down.
• Clients usually read mail through IMAP (or POP3) or through a provider's web and mobile API.

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Figure SDA-RWS-02-1 · Servers talk SMTP to each other; readers fetch with IMAP or POP · © forcsguy.com

Image: Tim Sheerman-Chase · CC BY 2.0 · Source · Wikimedia Commons: Old Post Sorting Office, Portsmouth

Architecture

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Figure SDA-RWS-02-2 · Email delivery architecture · © forcsguy.com

Sending

1. The client sends the message to its provider's submission server (with authentication).
2. The message is saved and put on an outbound queue, so slow recipients never slow the sender.
3. A worker finds the recipient domain's MX hosts, connects with SMTP, and delivers. The worker signs the message (DKIM).
4. Temporary failures are retried with backoff for days; permanent failures generate a bounce message.

Receiving

1. The incoming server accepts the connection and verifies the sender with SPF, DKIM and DMARC.
2. Filters score the message for spam, phishing and malware (rules, reputation, and machine learning).
3. The message is written to the user's mailbox: metadata in a scalable database, bodies and attachments in object storage, and an entry in the search index.
4. Clients are told (push, IMAP IDLE) that new mail arrived.

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Figure SDA-RWS-02-3 · Where a received message is stored and how readers are told · © forcsguy.com

Fighting spam and spoofing

Mechanism	What it does
SPF	The domain lists which servers may send for it
DKIM	The sender signs the message; receivers check the signature
DMARC	The domain says what to do when SPF or DKIM fail, and gets reports
Reputation	Track sending behaviour per IP and domain; rate-limit new senders
Content filters	Spam classifiers, link and attachment scanning

Storage design

• A mailbox is append-heavy and read by recent items. Partition by user id, so one user's mail sits together.
• Keep metadata (labels, flags, thread id, snippet) separate from large bodies and attachments.
• Threads group messages by headers (References, In-Reply-To) and subject.
• Search uses an inverted index per user, updated as mail arrives.
• Deduplicate identical large attachments with content hashes.
• Keep multiple replicas and backups: email is people's records.

Scale and reliability

• Queues absorb bursts, for example newsletter sends.
• Rate-limit outgoing mail per account to stop compromised accounts spamming.
• Spread incoming servers across regions; MX records can list several with priorities.
• Privacy: encrypt in transit (TLS between servers) and at rest, and control internal access carefully.

Real-world reading