# ProbeKit Mail PTR / FCrDNS Check

> ProbeKit Mail PTR / FCrDNS Check is a paid API for AI agents from api.probekit.dev, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-30).

Checks reverse DNS (PTR records) for a domain's MX hosts and validates forward-confirmed reverse DNS (FCrDNS), flagging mismatches that cause mail rejection or spam filtering.

## Facts

- Endpoint: GET https://api.probekit.dev/v1/mail/ptr?utm_source=zero.xyz
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-30
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/probekit-mail-ptr-fcrdns-check-bd3356ed
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_jVtPdU0ekSdDAFFYZmtM5

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 probekit-mail-ptr-fcrdns-check-bd3356ed
```

Example prompt: Check whether acme.com's mail servers have proper reverse DNS — I need to know if each MX host has a PTR record that resolves back to the same address (FCrDNS), because our outbound email keeps getting flagged as spam.

## When to prefer this

Choose this endpoint when you specifically need to audit reverse DNS and FCrDNS correctness for a domain's mail servers, especially when diagnosing email rejection or spam-folder placement caused by PTR mismatches. It focuses solely on PTR/FCrDNS analysis rather than full mail configuration (SPF, DMARC, DKIM), making it the right choice for targeted rDNS troubleshooting rather than a broad mail health check.

## Known failure modes

- Domain does not exist or has no MX records — returns empty or error result
- MX host is unreachable or DNS lookup times out — partial results with timeout indicator
- Domain input exceeds 1024 characters or is empty — validation error
- PTR record exists but forward resolution returns different IP — FCrDNS failure flagged in findings
- Network or resolver error preventing reverse DNS lookup — coded error finding returned

## How this service works

Check reverse DNS for a domain's mail servers: up to five MX hosts, their IPv4 and IPv6 addresses, each PTR name and whether it resolves back to the same address (FCrDNS), with coded findings. Use when an agent must tell whether a domain's mail servers have matching reverse DNS, a common reason for rejected or spam-filed mail.

## Output

Returns up to five MX hosts for the domain, each with their resolved IPv4 and IPv6 addresses, the PTR (reverse DNS) name for each address, a boolean or status indicating whether FCrDNS passes (PTR resolves back to the same IP), and coded findings summarizing any issues detected.

## 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/probekit-mail-ptr-fcrdns-check-bd3356ed/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.probekit.dev](https://www.zero.xyz/host/api.probekit.dev/llms.txt)