Gmail: say yes to the whole internet, and never lose a message

How an email service for 1.5 billion mailboxes works: an SMTP edge that refuses cheaply and answers 250 OK only after a quorum write in two zones, with message keys for retries and 451 deferrals as backpressure; a filter funnel that treats authentication as identity and lets reputation, content and user reports judge, on a classifier whose move to RETVec raised detection by 38 percent; SPF, DKIM, DMARC alignment and Gmail's 2024 rules for senders of more than 5,000 a day; metadata apart from content-addressed blobs, erasure-coded as they age, with slow purges, cell-by-cell rollouts and an offline copy after the 2011 tape restore; one search index per mailbox; and quotas and receiving limits that defer with temporary failures.