By the team at MailTester Ninja, a real-time email verification API that stores nothing.
Across 50,000 domains, mail hosting is strikingly concentrated:
- Top 1 provider handles 32.4% of mail-enabled domains
- Top 3 handle 83.4%
- Top 5 handle 92.1%, out of 20 distinct providers we detected
Leaders right now: Other / self-hosted (32.4%), Google Workspace / Gmail (28.3%), Microsoft 365 / Outlook (22.7%), Proofpoint (5.6%), Mimecast (3.1%).
For senders this cuts two ways. Most of your deliverability outcome is decided by a few gatekeepers, get right with them and you reach the majority of inboxes. But the long tail is where surprises live: a small self-hosted or regional gateway can silently greylist or block you with rules nothing else uses. You can't assume "it worked for Gmail" means "it works everywhere", which is why verifying against the actual receiving server matters.
Check any domain yourself, MailTester Ninja's free deliverability tools cover it end to end in one place: MX / SPF / DMARC analysis, an inbox-placement test scored out of 100, an SPF & DMARC generator and a blocklist checker, all with no signup and nothing stored. Need to confirm whether a specific mailbox actually exists and is deliverable? That is exactly what MailTester Ninja's email verifier does in real time, and we store no data.
Source: MailTester Ninja's open Email Infrastructure Index, a live DNS scan of 50,000 of the world's most-linked domains, updated daily, aggregate only. Free to cite under CC BY 4.0. Snapshot: Mon, 20 Jul 2026 04:18:17 GMT.
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