Enter any IPv4/IPv6 address (e.g. 8.8.8.8) or a domain name (to automatically inspect all of its MX mail server IPs) and verify both in-addr.arpa PTR resolution and Forward-Confirmed Reverse DNS (FCrDNS).
Botnets and residential spam proxies rarely control the reverse DNS zone (in-addr.arpa) of their IP address. Because only the IP owner (your cloud host or ISP, such as AWS, DigitalOcean, Hetzner, or Oracle Cloud) can set a PTR record, Gmail, Yahoo, and Microsoft Exchange reject mail from any IP lacking a valid PTR record or failing Forward-Confirmed Reverse DNS (FCrDNS).
# Using dig (Linux / macOS)
dig -x 8.8.8.8 +short
# Using host
host 8.8.8.8
# Using nslookup (Windows / Linux / macOS)
nslookup -type=PTR 8.8.8.8Running a reverse DNS lookup (also called a DNS reverse lookup, PTR lookup, or IP-to-hostname check) queries the special .in-addr.arpa (IPv4) or .ip6.arpa (IPv6) zone defined in RFC 1035 and RFC 1912. Every major inbound mail transfer agent — including Gmail (aspmx.l.google.com), Microsoft 365 Exchange Online Protection, Yahoo/Ymail (yahoodns.net), and Apple iCloud — performs an immediate PTR record lookup on your sending IP address the millisecond your server opens a TCP port 25 connection.
Having a PTR record is only half of the requirement: enterprise MTAs enforce Forward-Confirmed Reverse DNS (FCrDNS, codified in RFC 7001 / BCP 185). Under FCrDNS, your IP address (e.g., 198.51.100.42) must resolve via PTR record lookup to a canonical hostname (e.g., mta1.yourdomain.com), AND a forward DNS A/AAAA lookup on mta1.yourdomain.com must resolve back to 198.51.100.42. Furthermore, if your PTR record returns a generic cloud/ISP pattern such as ec2-198-51-100-42.compute-1.amazonaws.com or static-198-51-100-42.isp.net, Gmail and Outlook will reject your connection with SMTP 550 5.7.1 or 421 4.7.0.
| Reverse DNS / PTR Check Scenario | Example DNS Resolution Output | Gmail, Yahoo & Microsoft 365 MTA Verdict |
|---|---|---|
| Valid PTR Record + Matching FCrDNS + HELO Alignment | 198.51.100.42 → mta1.brand.com → 198.51.100.42 | ✅ PASS (Full IP reputation credit & inbox eligibility) |
| Missing PTR Record (NXDOMAIN on in-addr.arpa) | 198.51.100.42 → NXDOMAIN (No PTR record found) | ❌ HARD REJECT (SMTP 550-5.7.25 / 550 5.7.1 Missing PTR) |
| Forward-Confirmed Mismatch (Broken FCrDNS) | 198.51.100.42 → mail.brand.com → 203.0.113.99 | ❌ SPAM FOLDER / 421 DEFERRAL (Suspected IP spoofing) |
| Generic Cloud / Residential rDNS Hostname | 198.51.100.42 → vps-198-51.cloudprovider.net | ⚠️ THROTTLED (Flagged as unconfigured dynamic/VPS host) |