Inside fast mode: what happens in 40 ms

Email verification belongs on the critical path only when the latency budget is predictable. Fast mode never opens an SMTP connection; it answers from signals that resolve in tens of milliseconds and caches the expensive one.

$ mxping verify alice@gmail.com
-> Syntax OK (RFC 5322)
-> MX cache hit: gmail-smtp-in.l.google.com
-> Disposable list: not listed
-> Role-based: no
-> Score: 80 (medium — free provider)
OK valid 23ms

The MX lookup is the only network hop. Results are cached in-process for five minutes and in Redis for up to a day, so a list dominated by a few large providers mostly never leaves memory. Everything else — syntax, the disposable-domain list, role-account detection, the free-provider table — is a local lookup.

A mailbox-level probe (deep mode) is a different trade: slower, dependent on IP reputation, and priced accordingly. It ships when the proxy infrastructure behind it is ready, gated per plan.