Credit where it is due: Gmail is one of the better providers to verify against in principle. An address that does not exist returns 550-5.1.1 with a stable, parseable message. That is more than Yahoo, AOL, Proton or a Microsoft tenant with edge blocking disabled will give you.
The difficulty is getting the question asked. Google restricts connections from IP ranges it associates with abuse, and large swathes of cloud provider address space fall into that category. A probe from a default AWS or Google Cloud instance is frequently refused before it reaches the recipient stage.
When they do refuse, the response is a 421 or a dropped connection. Neither carries any information about the mailbox. A verifier that scores those as invalid is deleting working Gmail addresses, which on a consumer list is most of your audience.
There is a quieter Gmail problem too. Dots in the local part are ignored and everything after a plus sign is discarded, so j.smith@gmail.com, jsmith@gmail.com and jsmith+news@gmail.com are one mailbox. A list can carry the same person six times and no string comparison will notice.
What Gmail will never tell you
Whether the account is dormant. Google does not leak account state at the recipient stage, so an address abandoned in 2014 answers exactly like one checked hourly.
Whether the mailbox is over quota or suspended. Those surface at delivery, long after any probe has finished.
Whether the person will engage, which is the thing Gmail itself weighs most heavily when deciding where your message lands. That is in your own open and click data and nowhere else.
Where this argument costs us something
The short version
- Normalize Gmail addresses before verifying, so you stop paying for duplicates of one mailbox.
- Treat a 421 from Google as unknown rather than as a verdict.
- Expect a verifier's Gmail coverage to depend on its infrastructure rather than on its algorithm.
Questions people ask
Do dots in a Gmail address matter?
Not to Google. They are ignored entirely, so j.smith@gmail.com and jsmith@gmail.com reach the same inbox. They matter to your list, because both rows get billed by a verifier that does not normalize them.
Why did my Gmail addresses come back unknown?
Almost always because Google refused or throttled the connection rather than judging the address. That is a statement about the probe's origin, not about the mailbox.