Look up IPs
A hostname is resolved to its A and AAAA addresses. A domain with MX records is checked on its mail hosts, which is what receivers see.
How it works
Resolve
IPs are used as given. Hostnames resolve to A and AAAA; a bare domain also resolves its first three MX hosts.
Reverse
The PTR at in-addr.arpa or ip6.arpa is fetched, then the name is resolved forward to confirm it points back at the same IP.
Network
The ASN, prefix, country and registry come from Team Cymru's IP-to-ASN DNS zone; the AS name from the matching AS record. All over DNS-over-HTTPS.
Judge
Missing or unconfirmed rDNS, auto-generated host names, multiple PTRs and consumer networks are each flagged with what to do.
What a receiver sees first
Before a mail server reads a single header, it has the connecting IP. It looks up the reverse DNS, resolves that name forward, and checks whether the address is in a network it has opinions about. Gmail's documented requirement is a PTR that forward-confirms; without one it refuses the connection. Microsoft defers. Spamhaus's PBL lists entire consumer ranges that should never send mail directly. All of that happens in the first second, and none of it is visible in the sending tool. This lookup shows you the same view.
Forward-confirmed reverse DNS
A PTR alone is not enough; anyone with a /24 can write whatever they like into the reverse zone. Forward confirmation means the name in the PTR resolves back to the IP that claimed it, which proves the same party controls both. The check is FCrDNS pass or fail in the table. On Google Workspace and Microsoft 365 this is always fine because the provider's IPs are the provider's problem. It matters when someone runs their own SMTP, a relay, or a VPS with a sending tool on it.
The ASN tells you the neighbourhood
An IP on Google's or Microsoft's AS carries their reputation, which is why cold email on Workspace inboxes lands at all. An IP on a cheap VPS provider carries that provider's reputation, which is shared with every other customer on the same range and is usually poor. An IP on a consumer ISP is blocked almost everywhere. The AS name and the type guess in the table are the fastest way to see which of those you are dealing with, whether it is your own server or the origin IP in a header you are investigating.
Generic reverse names
203-0-113-5.static.example-hosting.net is technically a valid PTR that forward-confirms. Filters still score it, because that pattern is what an unmanaged host looks like, and unmanaged hosts are where compromised machines send from. If you run your own sending server, the PTR should be the hostname the server uses in HELO — mail.acme.co — and that name should have an A record back to the IP. The tool flags the auto-generated pattern so you know to fix it.
Frequently asked questions
What is FCrDNS?
Forward-confirmed reverse DNS: the IP's PTR names a host, and that host's A or AAAA record includes the IP. Gmail requires it for direct SMTP delivery.
Where does the ASN data come from?
Team Cymru publishes IP-to-ASN mappings as DNS TXT records, which this tool queries over DNS-over-HTTPS. No third-party API and nothing leaves the browser except DNS queries.
Why does my Google Workspace mail show a Google IP?
Because Workspace sends from Google's servers. The IP and its reverse DNS are Google's; the reputation that is yours is the domain's.
Does a bad PTR affect cold email from Workspace or Microsoft 365?
No. The providers manage their own PTRs. It affects anyone sending through their own SMTP server, a private relay, or a VPS.
The tool says 'generic host' — is that a problem?
It is a signal, not a block. Set the PTR to a real mail hostname with a matching A record and it goes away.
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