Providers
How well an address checks out depends mostly on who runs the mailbox, not on who does the checking.
Yahoo and AOL say yes to every address on purpose. It stops people harvesting their user list. Gmail and Microsoft 365 block checks from cloud servers, and slow down the rest.
So these pages say which providers we read well, and which ones nobody can read.
AOL Mail email verification
AOL Mail runs on Yahoo's servers, so aol.com and aim.com accept every recipient at the SMTP stage and mailbox-level verification is not possible for anyone.
AT&T Mail email verification
AT&T Mail is run by Yahoo, so att.net, sbcglobal.net, bellsouth.net and the other legacy domains accept every recipient and can't be verified by mailbox.
BellSouth email verification
BellSouth's bellsouth.net mail is run by Yahoo through AT&T, so the server accepts every recipient at the SMTP stage and the domain classifies as accept-all.
BOL email verification
BOL, the free Brazilian webmail on UOL's platform, returns a 550 for unknown bol.com.br recipients, but foreign connections are limited and timeouts common.
Charter Spectrum email verification
Charter Spectrum runs charter.net, spectrum.net, roadrunner.com and twc.com. Unknown recipients get a 550 on the main domains, and canceled accounts lapse.
Comcast email verification
Comcast returns a 550 for unknown comcast.net recipients but blocks bulk-looking traffic at the connection. Mailboxes on a canceled Xfinity plan can lapse.
Cox email verification
Cox moved cox.net mailboxes to Yahoo Mail in 2024, so Yahoo's servers now accept every recipient. No new cox.net addresses have been issued since August 2019.
Daum Mail email verification
Daum Mail, part of Kakao, serves daum.net and hanmail.net from the same hosts. Replies are slow and throttled from abroad, so timeouts are common.
Fastmail email verification
Fastmail rejects unknown recipients with a plain 550 and rate-limits politely, so it verifies well. Masked Email aliases and wildcard domains are the catch.
Free email verification
Free's free.fr and aliceadsl.fr mailboxes return a 550 for unknown recipients, but its connection throttling is severe, so a large batch needs slow pacing.
Gmail email verification
Gmail rejects nonexistent addresses with a 550-5.1.1 reply, but it blocks or tarpits SMTP probes from most cloud IP ranges, so results depend on the origin.
GMX email verification
GMX, United Internet's free German webmail, rejects unknown recipients with a 550. Strict per-IP connection limits mean probes must be paced to get answers.
Google Workspace email verification
Google Workspace domains reject unknown addresses as Gmail does unless the admin sets a catch-all or routing rule, which is why many classify as catch-all.
Hotmail email verification
Hotmail addresses route into Outlook.com and inherit its throttling and inconsistent replies. Microsoft still offers hotmail.com at sign-up, so age varies.
iCloud Mail email verification
iCloud Mail rejects unknown addresses with a clear 550, so icloud.com, me.com and mac.com verify well. Hide My Email aliases can be switched off at any time.
Libero email verification
Libero, run by Italiaonline, returns a 550 for unknown recipients in normal conditions. During its outages every probe fails temporarily and reads as unknown.
Live.com email verification
Live.com addresses from the Windows Live years route into Outlook.com with the same throttling. Many are Microsoft sign-in names that verify but go unread.
Mail.ru email verification
Mail.ru serves mail.ru, inbox.ru, bk.ru and list.ru from one platform. It rate-limits hard and often refuses foreign IP ranges, so many results are unknown.
Microsoft 365 email verification
Microsoft 365 rejects unknown recipients by default with 550 5.4.1. Internal relay routing or a gateway like Mimecast can still make a tenant a catch-all.
MSN email verification
MSN is Microsoft's oldest consumer mail domain. Its addresses still route into Outlook.com with the same throttling, and most on a current list are very old.
Naver Mail email verification
Naver Mail rejects unknown naver.com recipients with a 550, but it rate-limits foreign connections, so part of any large Korean batch comes back unresolved.
NetEase 126 email verification
NetEase's 126.com shares servers and rate limits with 163.com, and probes from outside China almost never get an answer. The two hold separate accounts.
NetEase 163 email verification
NetEase's 163.com usually refuses, resets or times out probe connections from outside China, so its addresses come back unknown for any foreign verifier.
Onet email verification
Onet returns a consistent 550 for unknown recipients on onet.pl, op.pl, vp.pl and poczta.onet.pl. Each domain holds separate mailboxes, so don't merge rows.
Orange email verification
Orange runs orange.fr and the older wanadoo.fr on the same hosts. It returns a 550 for unknown recipients and limits foreign probes. laposte.net isn't Orange.
Outlook.com email verification
Outlook.com covers outlook.com, hotmail.com, live.com and msn.com. Its throttled, inconsistent SMTP replies can give one address two codes in an hour.
Proton Mail email verification
Proton Mail does not document how its servers answer a recipient probe, and public reports disagree, so probing alone can't yet confirm a Proton address.
QQ Mail email verification
QQ Mail, Tencent's service for qq.com and foxmail.com, drops most connections from outside China, so a probe usually gets no answer instead of a verdict.
Rediffmail email verification
Rediffmail usually returns a 550 for unknown recipients when the connection completes. Most rediffmail.com addresses are old accounts that still accept mail.
Seznam email verification
Seznam serves seznam.cz, email.cz and post.cz from the same hosts and returns a clean 550 for unknown recipients, so Czech consumer lists verify well.
Sina Mail email verification
Sina Mail serves sina.com and sina.cn from its freemx hosts and vip.sina.com from separate ones. Connections from outside China are usually dropped.
T-Online email verification
T-Online, Deutsche Telekom's mail service, returns a dependable 550 for unknown recipients but refuses hosts that lack matching forward and reverse DNS.
Terra email verification
Terra answers probes for terra.com.br and terra.com inconsistently, with frequent temporary failures. Some regional Terra domains no longer publish an MX.
UOL email verification
UOL is Brazil's paid portal mail, so uol.com.br addresses are likelier to be maintained than free webmail. Unknown recipients get a 550 when probes complete.
Verizon Mail email verification
Verizon moved verizon.net mail to AOL, which runs on Yahoo's servers, so these addresses accept every recipient at the SMTP stage and classify as accept-all.
Web.de email verification
Web.de, run by United Internet alongside GMX, returns a consistent 550 for unknown recipients. The two brands share strict connection limits.
WP.pl email verification
WP.pl serves wp.pl, o2.pl and tlen.pl under Wirtualna Polska and returns a stable 550 for unknown recipients, with moderate rate limits on probes.
Yahoo Mail email verification
Yahoo Mail answers 250 or 252 to every recipient to stop address harvesting, then bounces later, so no verifier can confirm a yahoo.com mailbox over SMTP.
Yandex Mail email verification
Yandex Mail rejects unknown consumer addresses with 550 5.1.1 but throttles per IP, and Yandex 360 business domains can enable a catch-all.
Zoho Mail email verification
Zoho Mail returns 550 5.1.1 for unknown recipients on most setups, so it verifies well. Admin-enabled catch-alls and abandoned free domains are the limits.
Run a sample through it
100 free checks a month, no card, and the unknowns come back labeled.