# Lookproof PTR / FCrDNS Mail Server Check

> Lookproof PTR / FCrDNS Mail Server Check is a paid API for AI agents from api.lookproof.dev, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-31).

Checks reverse DNS (PTR records) for a domain's MX hosts and validates full-circle reverse DNS (FCrDNS) for each mail server IP address

## Facts

- Endpoint: GET https://api.lookproof.dev/v1/mail/ptr?utm_source=zero.xyz
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-31
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/lookproof-ptr-fcrdns-mail-server-check-18365b90
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_90VWUuOBcWF_gdVgqusOm

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability lookproof-ptr-fcrdns-mail-server-check-18365b90
```

Example prompt: Can you check whether the mail servers for acme.com have proper reverse DNS — I want to know if each MX host's IP has a PTR record that resolves back to the same address, since some of our emails are getting rejected.

## When to prefer this

Use this endpoint when you specifically need to diagnose reverse DNS configuration for a domain's mail servers, especially when troubleshooting email rejection or spam-filing caused by missing or mismatched PTR records. Prefer this over generic DNS tools when you need FCrDNS validation (PTR resolves back to the original IP) across all MX hosts in a single call.

## Known failure modes

- Domain does not exist or has no MX records — returns empty or error result
- Domain has more than five MX hosts — only up to five are checked
- PTR lookup fails due to no reverse DNS zone configured — reported as missing PTR
- Network timeout resolving IPv4 or IPv6 addresses — partial results may be returned
- Invalid domain format provided — returns validation error

## How this service works

Check SPF, DMARC, DKIM, MTA-STS, DNSSEC and more, with results that explain what was found and why. Free browser checks and a pay-per-request API with x402.

## Output

Returns up to five MX hosts for the domain, each with their resolved IPv4 and IPv6 addresses, the PTR name found for each address, whether that PTR resolves back to the same IP (FCrDNS pass/fail), and coded diagnostic findings indicating configuration problems.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET",
      "HEAD",
      "DELETE"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "title": "DomainInput",
     "required": [
      "domain"
     ],
     "properties": {
      "domain": {
       "type": "string",
       "title": "Domain",
       "examples": [
        "example.com"
       ],
       "maxLength": 1024,
       "minLength": 1
      }
     },
     "additionalProperties": false
    }
   },
   "additionalProperties": false
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/lookproof-ptr-fcrdns-mail-server-check-18365b90/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.lookproof.dev](https://www.zero.xyz/host/api.lookproof.dev/llms.txt)