Problem Understanding
Restate the problem in your own words.
Design an Email Service (Gmail / Outlook)
Design a Gmail / Outlook-class email service: accept inbound SMTP from any sender with DKIM + SPF + DMARC validation, filter spam + viruses, store per-mailbox in chunked storage, thread by Subject / References, support folder + label views, do full-text search across multi-GB mailboxes, and sync to thousands of client devices via IMAP IDLE or push. The architecture is split across a hot SMTP ingest tier, a heavy spam-filter pipeline, a per-mailbox storage tier with full-text indexing, and a sync gateway for clients. Newsletter blasts (1 sender → 10 M recipients in minutes) and DSN bounce storms are the operational nightmares; spam false-positives are the existential UX risk.
- Gmail~1.8 B users; petabytes of mail; the industry standard for spam filtering.
- Outlook / Microsoft 365Enterprise-focused; same architecture + on-prem options via Exchange Server.
- Yahoo Mail / AOLOlder entrants; same shape, smaller scale.
- ProtonMail / TutaPrivacy-first; E2E-encrypted at rest; same SMTP + IMAP frontend.
Your task: read the problem above, then write what the system is, who uses it, the rough scale, and the headline UX expectation — in your own words. Submit for AI review when you're ready.
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