# Design Gmail: system design interview question

> Global email service: reliable delivery, large attachments, conversation groups, and mailbox state across devices.

Source: https://sysround.com/questions/design-gmail
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design an email service for users worldwide. Users must send and receive messages, view conversations, and keep mailboxes current across devices. The service must support large attachments."

## What to clarify first

- Interfaces and scope
- Users and message volume
- Read and write mix
- External delivery
- Message size and retention
- Latency
- Consistency and availability
- Regional failure and durability
- Conversation and device behavior
- Security and abuse

## Scope of a strong answer

Core requirements:

- Send and receive mail within the service and with other providers
- Show messages in conversations
- Synchronize messages, labels, read flags, and deletions across devices
- Transfer and retain large attachments

Usually out of scope:

- Calendar features
- POP and IMAP support
- Spam classifier internals
- Account creation and recovery

## What interviewers probe

### Message acceptance and delivery

An acceptance response creates a durability promise. External SMTP delivery can have an unknown result, and each recipient can reach a different state.

### Mailbox state across devices

Devices miss notifications and return after long absences. Concurrent changes and deletions must converge without gaps in the mailbox history.

### Conversation membership

Reply headers can be absent, late, or false. Different recipients also see different parts of the same conversation.

### Attachment transfer and lifetime

Attachments dominate storage and transfer volume. Interrupted uploads, shared references, and region failures can leave messages with missing content.

## Common mistakes

- Promises exactly-once SMTP delivery despite an unknown remote result.
- Acknowledges a message before both content and delivery work meet the durability requirement.
- Uses Message-ID as a trusted unique key or the subject as the only conversation rule.
- Uses push notifications as the only mailbox synchronization mechanism.
- Stores attachments in mailbox rows or deletes shared content with one mailbox entry.
- Adds regional replicas without explaining content durability, writer ownership, and failover.

## The bar by level

- **Senior:** Defines acceptance boundaries, recipient states, and idempotency. Designs gap-free mailbox sync with deletion history. Handles conversation privacy and attachment lifetime.
- **Staff+:** Proves regional durability for metadata and content. Defines writer failover and ambiguous SMTP outcomes. Quantifies replication cost, provider backlogs, and large recipient fan-out.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=email-service


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.