Verifying Gmail addresses

• A live address returns 250 at RCPT TO and nothing else happens. Google does not leak whether the account is dormant, suspended or over quota at this stage.
• A dead address returns 550-5.1.1 with the phrase "The email account that you tried to reach does not exist" and a link to a support page. That string is stable enough to parse.
• Probe from an address range Google dislikes and you get 421-4.7.0 instead, which says nothing about the mailbox. A verifier that scores that as invalid is inventing data.
• Google also applies per-connection rate limits that tighten as the hour goes on. The first few hundred probes behave differently from the next few thousand.

• Since February 2024 anyone sending more than 5,000 messages a day to Gmail needs SPF, DKIM, DMARC, one-click unsubscribe and a spam complaint rate under 0.3%.
• Google Postmaster Tools is the only place you see Gmail's own data on your mail, such as spam rate and compliance status, and it needs a verified domain plus enough volume to report anything.
• Gmail weighs engagement more heavily than most providers. Addresses that exist but never open are a deliverability problem no verifier can find for you.

Say you export 6,200 signups from a webinar form and 2,480 of them end in gmail.com. Before any server gets asked anything, read the local parts. Google's sign-up form allows only letters, numbers and periods in a Gmail username, and it asks for 6 to 30 characters. That means jen_park@gmail.com and mike-t@gmail.com almost certainly were never issued to anyone. Somebody typed them wrong, or typed them on purpose to get past your form.

You can catch those rows with a one-line rule and spend nothing on them. The same trick doesn't work for a misspelled domain. Addresses at gmial.com or gmai.com belong to a different domain entirely, and many look-alike domains are registered and do accept mail, so an MX check alone won't flag them. A typo list handles those better than DNS does.

Google says a deleted Gmail username can't be registered again, by you or by anyone else. That makes a 550 from Gmail unusually durable. At providers that hand old names to new owners, an invalid row can turn valid again under a stranger. Here it won't, so you can suppress the address for good and skip the re-check.

Valid results age in the other direction. Since December 2023 Google has been deleting personal accounts that go two years without a sign-in, and the policy covers consumer accounts, not Workspace ones. If a segment of your list hasn't opened or clicked since 2023, some of those mailboxes have likely been removed since you last verified. Re-run that segment before a win-back campaign instead of trusting an old clean result, because the first send to a pile of deleted accounts is exactly the bounce spike Gmail's filters notice.