Gmail and Google Workspace answer RCPT TO with a 250 or a 550 like any other mail server. The catch is where you probe from: they block SMTP probes from cloud IP ranges, so a script on a rented server often gets no verdict at all. Our Gmail provider page and Google Workspace page cover the details.
Microsoft is harder. Outlook and Microsoft 365 servers rate-limit probe traffic aggressively and don't always answer the same way twice. A burst of checks against one company can turn clean replies into 4xx deferrals halfway through a batch. We wrote up the pattern in how Microsoft rate-limits verifiers.
Blocking doesn't make an address bad. A refused or deferred probe means the server declined to answer, so the right result is unknown. A verifier that turns those rows into invalid makes you delete real people, and one that turns them into valid pushes your bounce rate up.
ZapBounce runs its own SMTP check on port 25 and records what the server actually said. A clean 250 or 550 becomes valid or invalid. A block comes back unknown with the reason probe_blocked, a deferral comes back unknown with greylisted or throttled, and every row carries the literal smtp_code. Unknown rows are never billed, so a refused probe costs you nothing.
Re-check unknown Microsoft rows in a later, slower pass, since rate limits reset. For Gmail rows that stay unknown, let your own engagement data decide: an address that replied last month is live. Try one address in the free email verifier, or see ZapBounce pricing, from $5 for 1,000 checks.
Related questions
Does Gmail allow SMTP email verification?
Gmail answers RCPT TO with standard reply codes, and it blocks SMTP probes that come from cloud IP addresses. A probe it refuses gets no verdict, and ZapBounce returns that row as unknown at no charge.
Why do Outlook addresses come back unknown?
Microsoft rate-limits probe traffic hard and answers inconsistently, so a check can end in a 4xx deferral. That is a temporary answer, not a verdict. Re-check the unknown rows in a slower pass later.
Is SMTP email verification dead?
No. Mail servers still answer RCPT TO with a 250 or a 550, and that reply is the only direct test of a mailbox. Gmail and Microsoft 365 add friction by blocking cloud probes and rate-limiting, which a verifier handles by reporting unknown instead of guessing.
Does a blocked verification probe cost a credit?
Not at ZapBounce. A blocked, throttled or timed-out probe returns unknown, and unknown is never billed. You pay one credit only when a mail server gave a real answer.