Best email verifiers for Microsoft 365
Microsoft 365 throttles verification probes and some tenants accept every address. Eight verifiers ranked on how they report and bill those results.
How this was compared
Disclosure: ZapBounce is our product. Figures were read from each vendor's own pages in September 2026, and the criteria below were applied to every entry, ours included.
The short version
- What you pay when Microsoft won't answer: whether an unknown result from a throttled or blocked probe spends a credit.
- What the result tells you: the reason, the SMTP reply and the host that answered, so a Mimecast or Proofpoint gateway in front of a tenant is visible.
- Catch-all reporting: whether a tenant set to accept every address comes back as its own result instead of a confident valid.
- Credit expiry and the published price at 10,000 addresses, read from each vendor's own pages in September 2026.
- Disclosure: ZapBounce is our product, and the same criteria were applied to it.
At a glance
| 10,000 addresses | Credit expiry | Bills unknowns | |
|---|---|---|---|
| ZapBounce | $25 | Credits never expire | No |
| Kickbox | $70 | Credits expire one year from purchase | No |
| Verifalia | $49.90 at Standard quality | Purchased credits never expire; daily free credits expire each day | Not stated |
| Bouncer | $60 | Credits never expire | No |
| Emailable | $0.0076 an address at the 5,000 entry, 15% off on a subscription | Credits never expire | No |
| MillionVerifier | Not shown in the tiers we could read | Credits never expire, stated twice on their page | No |
| Clearout | Not listed | Credits never expire and roll over | No |
| ZeroBounce | Subscription only | Credits roll over while the subscription is active | Not stated |
Vendor figures read from their own pricing pages in September 2026. Confirm before purchase, because prices move.
The ranking
Best first, with the situation each one fits.
ZapBounce
Disclosure: ZapBounce is our product. Our own SMTP check talks to the tenant's mail host on port 25 and closes before any message is sent. Each row carries the result, a reason such as mailbox_not_found or throttled, the literal smtp_code and the mx_host that answered.
- Best for:
- B2B lists where most contacts sit on company domains hosted in Exchange Online, at $25 for 10,000 with unknowns never billed. The Microsoft 365 provider notes explain what each answer means, and how it works walks through the check.
Kickbox
Kickbox charges only for definitive results, so an address it could not resolve is refunded, and its Sendex score rates each address.
- Best for:
- Teams who want a quality score beside each Microsoft 365 contact.
- Less suited to:
- Credits bought ahead of need, since they expire one year from purchase (read 24 September 2026).
Verifalia
Verifalia reports why an address failed and sells three quality levels, costing 1, 2 and 4 credits an address, so you choose how hard to push on difficult servers.
- Best for:
- Developers who want to pay for a deeper check on hard domains.
- Less suited to:
- Deep cleans on a fixed budget, since High quality costs 2 credits an address and Extreme costs 4.
Bouncer
Bouncer never charges for duplicates or unknowns, and its credits never expire.
- Best for:
- Mixed lists of company and consumer addresses with risky domains to flag.
- Less suited to:
- Large lists on a tight budget, where its 100,000 tier costs $400.
Emailable
Emailable refunds duplicates and unknowns, and its credits never expire.
- Best for:
- Company lists over 5,000 contacts cleaned in one pass.
- Less suited to:
- Small lists, since purchases start at 5,000 credits.
MillionVerifier
MillionVerifier refunds any address it could not verify with certainty, and its credits never expire.
- Best for:
- Very large lists, with 1,000,000 checks at $449 (read 24 September 2026).
- Less suited to:
- Teams who want a monthly plan, since it sells pay-as-you-go only.
Clearout
Clearout attempts to resolve catch-all domains rather than reporting them, and gives 100 free credits with no card.
- Best for:
- Company lists where many domains accept every address.
- Less suited to:
- Pricing a clean up front, since its tier prices are not listed on its pricing page.
ZeroBounce
ZeroBounce pairs verification with activity data and a deliverability suite.
- Best for:
- Teams who want engagement data beside each company contact.
- Less suited to:
- Pay-as-you-go buyers at 10,000 checks, which ZeroBounce sells as a subscription only.
Why ZapBounce ranks first
How to choose
- Throttled rows: Kickbox, Bouncer, Emailable and MillionVerifier refund or skip unresolved addresses. ZapBounce never bills an unknown and names the reason, so you can re-check throttled rows a few days later on bulk.
- Depth of check: Verifalia charges 2 or 4 credits an address for its deeper levels. ZapBounce charges one credit for a valid, invalid or catch_all result, whatever the tenant does.
- Catch-all tenants: Clearout attempts to resolve catch-alls. ZapBounce labels them catch_all after one probe, so you send to them as a separate, slower segment.
- Credits: Kickbox credits expire one year from purchase (read 24 September 2026). ZapBounce credits never expire.
Questions people ask
Does email verification work on Microsoft 365 addresses?
Yes, for most tenants. A domain fully hosted in Exchange Online rejects unknown recipients with 550 5.4.1, which is a clear invalid. Tenants set to Internal relay or behind a gateway accept everything and come back as catch_all.
Why do Microsoft 365 addresses come back unknown?
Exchange Online throttles connections and refuses probes from IPs it has not built a history with. The check ends without a straight answer, which ZapBounce reports as unknown with the reason throttled or probe_blocked, and never bills.
Why do two Microsoft 365 domains give different results?
Recipient checks depend on how each tenant routes mail. One rejects unknown addresses at the edge, while the next relays to an on-premises server or a security gateway that accepts every address.
Are Outlook.com and Hotmail addresses the same as Microsoft 365?
No. Outlook.com, Hotmail and Live addresses are Microsoft's consumer service, while Microsoft 365 hosts company domains in Exchange Online. ZapBounce flags the consumer ones with free_provider set to true.
Run the same sample through two of them
100 free checks a month, no card. Compare how many addresses each one declined to judge, which matters more than either accuracy claim.