Gmail HLD: Storing Billions of Mailboxes and Threading Conversations

Email looks simple from the outside — you get a message, you read it, you reply. Inside, a service like Gmail is juggling three hard problems at once: never lose a message (durability above all), show a mailbox instantly even when it holds a hundred thousand messages, and group scattered replies into one tidy conversation. That last one — conversation threading — is the most interview-worthy piece, because it turns out to be a graph-connectivity problem hiding behind a few email headers. This is the inside of Gmail / Outlook / any mail service. The signature problem is threading: deciding whic…