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Design an Email Service (Gmail)

Design an email service like Gmail handling 1.8B users storing 500EB of email, accepting ~300B inbound messages per day from the public SMTP network while filtering 90%+ as spam, and serving full-text search over a user's entire inbox in sub-200ms. The interview centerpiece is the asymmetric architecture: SMTP is an untrusted public protocol with hostile traffic patterns (spam, phishing, sender forgery) that needs heavy gateway-side filtering, while the user-facing IMAP/web layer needs cheap reads, pagination of huge mailboxes, and per-user inverted indexes for search. We cover the SMTP MX gateway, the spam pipeline (SPF/DKIM/DMARC + ML), the per-user inverted index for search, and how mailboxes scale when one user holds 50GB of email.

Explain semantic and exact-match retrieval

Explain how embeddings and exact-term matching find candidates for a query, and why the top result is a ranking signal rather than a verified answer.

Compare lexical and dense retrieval on fixtures

Rank four help articles by shared words and by cosine similarity, score both rankings against labels, and explain a dense miss and an accidental lexical success.