Free tool · IPv6 deliverability
Is your mail server IPv6-ready?
Gmail, Yahoo, and Microsoft 365 all weight IPv6 posture for inbox placement, and Gmail outright rejects IPv6 mail without valid reverse DNS. Most MX checkers ignore AAAA entirely; we surface the real deliverability gap.
What this tool checks
IPv6 is no longer "future infrastructure." Gmail rejects inbound IPv6 mail without valid reverse DNS outright (the classic 550-5.7.1 rejection). Yahoo and Microsoft 365 weight it heavily for inbox placement. The problem: most MX checkers only resolve A records and never look at AAAA, so the IPv6 posture stays invisible until a deliverability ticket lands and nobody can explain why Gmail keeps bouncing every third message.
This checker resolves your domain's MX records, then for each MX host looks up AAAA. For every AAAA address it performs a Forward-Confirmed reverse DNS check (resolve PTR, then forward-resolve that hostname's AAAA, then confirm the original IP appears in the result). FCrDNS is the standard the big mailbox providers apply; passing it is the difference between "delivered" and "drop on the floor."
We also check the apex A and AAAA so you can see whether the rest of the infrastructure is v6-ready. An apex with AAAA but no MX-side AAAA is an inconsistent posture worth calling out.
How to read the results
Grade A: every MX host has AAAA, every AAAA address has valid FCrDNS, the apex has AAAA. Modern providers will treat your mail as fully IPv6-capable.
Grade B: MX-side IPv6 is fully clean; the apex isn't on IPv6 yet. Fine for mail flow; affects only the web side.
Grade C: MX hosts have AAAA but one or more fails FCrDNS. Gmail will defer or drop IPv6 mail from those hosts. Fix the reverse DNS at whoever controls the IP block (often the mail host's support team via a ticket).
Grade D: only some MX hosts have AAAA - inconsistent posture. Mail-flow routing on IPv6 backups becomes unpredictable. Either roll out AAAA everywhere or remove the partial v6 to make the posture consistent.
Grade F: no IPv6 on any MX host, but the apex publishes AAAA. The rest of the infrastructure is v6-ready, your mail isn't. Worth investigating - it's usually a missing AAAA on the mail-server's own hostname.
N/A: no IPv6 anywhere, or no MX records. Not "broken," just not v6-ready. Modern mailbox providers increasingly weight v6 for inbox placement; consider rolling out AAAA on MX hosts.