Design Gmail
Hard · System Design · about 45 minutes
Asked at Google, Microsoft, Apple, Salesforce.
Design an email service like Gmail for ~1.5B users. Users send and receive mail with the rest of the internet over SMTP, read threaded mailboxes on web, mobile and IMAP clients, organize with labels, and search years of mail in under a second. Most inbound mail is spam, and an accepted message must never be lost.