Domain Mail Check in ChatGPT

Questions it answers

Check how a domain is set up for email, whether Gmail and Outlook are likely to accept its mail, and who it is registered with, from public DNS, free blocklists and registry data.

Ask "will Gmail accept mail from example.com sent from 203.0.113.7?", "is DMARC set up correctly for github.com?", "check SPF and DKIM for example.com", "which mail provider does acme.io use?" or "when does openai.com expire?".

The email check reads the domain's SPF record (following include and redirect, counting DNS lookups against the limit of 10), its DMARC policy, the MX records and, when you give a DKIM selector, whether the DKIM key is published. It returns a grade from A to F for protection against spoofing, the problems it found and a short list of fixes. The DNS records tool reads A, AAAA, MX, TXT, CAA and NS records with their TTLs, which also shows which mail provider a domain uses and which certificate authorities may issue for it. The registration tool reads the domain's registry record over RDAP: registrar, creation and expiry dates, status flags and name servers.

The delivery check gives one verdict for Gmail and Outlook, the top 3 causes and one fix for each. It tests the published sender rules (SPF listing the sending IP, DKIM, DMARC, reverse DNS of the IP, key length) and reads 6 free blocklists (SpamCop, PSBL, UCEPROTECT level 1, Mailspike and NordSpam for the IP and the domain). Each rule and each list names its official source.

The cold-email launch score gives a score from 0 to 100 for a new sending domain, with a verdict of ready, almost or not_ready and the 5 fixes to make first. It tests SPF, DKIM, DMARC, reply mail, a domain blocklist, the domain's age from the registry record and the shape of its name. Each rule names its source and is labelled standard (a published requirement) or opinion (common advice nobody requires, which never decides not_ready).

The DNS hygiene check finds 4 faults beyond SPF, DKIM and DMARC: no CAA record (any certificate authority may issue for the domain), DNSSEC that is off or broken, mail servers that do not offer STARTTLS (mail arrives unencrypted), and dangling CNAMEs on 20 common subdomains such as www, shop and help, which point at a name that no longer exists, some of which anyone can register or create. For STARTTLS it reads only the greeting and EHLO answer of up to 2 mail servers and never sends mail; a server it cannot reach is reported as not read, not as a fault.

DNS answers come from Cloudflare's DNS over HTTPS service with Google Public DNS as a fallback, and registration data from the registry of the domain's ending, found through IANA's RDAP directory. Every answer names its source and the date it was read.

It does not send a test message or read mailboxes, so a good verdict means no published rule is broken, not a promise of the inbox; it does not read sender reputation, discover subdomains beyond a fixed list of common names, or return who owns a domain: registrant and contact details are never read or returned. It checks the exact domain you give, so a parent domain's SPF or DMARC record is not inherited. The domains you ask about are not stored; the only lasting record is a daily count of calls per tool.

Short questions this server answers, each with a direct answer: Email SPF, DKIM, DMARC and Domain Expiry Check.

The idea behind it, in one live picture: how Domain Mail Check works.

What it can do

• Reads a domain's SPF record, counts its DNS lookups and flags weak or broken policies
• Reads the DMARC policy, percentage and report address
• Checks whether DKIM keys are published for up to 5 selectors you name
• Grades email spoofing protection from A to F with a short list of fixes
• Reads A, AAAA, MX, TXT, CAA and NS records with TTLs and names the mail provider from the MX records
• Shows registrar, creation and expiry dates, status flags and name servers from the registry over RDAP
• Never returns registrant or contact details; names its sources and dates
• Gives one verdict on whether Gmail and Outlook will likely accept the domain's mail, the top 3 causes and one fix each
• Checks SPF for the sending IP, reverse DNS, DKIM key length and DMARC against the Gmail, Yahoo and Outlook rules
• Reads 6 free blocklists for the domain and the sending IP, with each list's terms link
• Scores a cold-email sending domain 0 to 100 from SPF, DKIM, DMARC, domain age and name, with fixes
• Labels each rule standard or opinion and links its source, so advice is not shown as a requirement
• Finds a missing CAA record, off or broken DNSSEC, mail servers without STARTTLS and dangling CNAMEs

Tools

• launch_score, Score a cold-email domain. Use this when the user asks whether a new or secondary domain is ready for cold email or outreach, or wants a score for a sending domain, such as "is acme-mail.com ready for cold email?". Pass the domain and, optionally, up to 5 DKIM selectors. Returns a score from 0 to 100, a verdict (ready, almost, not_ready or incomplete), the 5 fixes to make first, every rule with its status, its kind (standard for a published requirement, opinion for common advice nobody requires) and its source link, the domain's registration date and age, what was not tested, and the source and as_of date. Opinion rules change the score but can never make the verdict not_ready. It reads public DNS and the public registry record only: it sends no message and cannot see sender reputation or warm-up, so it does not promise inbox placement.
• delivery_check, Check mail delivery. Use this when the user asks whether mail from a domain will reach Gmail or Outlook, or why it is rejected or goes to spam, such as "will Gmail accept mail from example.com sent from 203.0.113.7?". Pass the domain, the sending IPv4 address if known, the return-path domain (mail_from) if known and, optionally, up to 5 DKIM selectors. Returns one verdict (likely_accepted, at_risk, likely_rejected or incomplete) with a verdict per provider, the top 3 causes each with one fix, the exact SPF and DMARC records to paste (records_to_paste) and the DKIM steps for the detected mail provider (fix_steps), every rule checked with its official source link (SPF for the IP, DKIM, DMARC, reverse DNS, blocklists), the 6 blocklists with their terms links, what was not checked, and the source and as_of date. Gmail and Outlook are always both reported, so there is no provider argument. It reads public DNS and free blocklists only: it sends no test message and cannot see sender reputation, so it does not promise inbox placement.
• check_dns_hygiene, Check DNS security faults. Use this when the user asks about a domain's CAA record, DNSSEC, whether its mail servers offer STARTTLS, dangling CNAMEs or subdomain takeover risk, such as "does example.com have DNSSEC and CAA?" or "is any subdomain of acme.co.uk dangling?". Pass the domain. Returns 4 checks, each pass, fail, unknown or skipped with a detail and a fix: caa (the records), dnssec (signed, unsigned or broken), starttls (a result per mail server) and dangling_cnames (of 20 common names, CNAMEs to a missing name, each registrable, service or stale), a faults list worst first, and the source and as_of date. It reads public DNS and only the EHLO answer of up to 2 mail servers: it sends no mail, and it never registers or claims a name to test a takeover.
• check_email_auth, Check email authentication. Use this when the user asks whether a domain's email is set up correctly, such as "is DMARC set up for github.com?" or "check SPF and DKIM for example.com". Pass the domain and, if known, up to 5 DKIM selector names. Returns SPF (policy, DNS lookup count, issues), DMARC (policy, percentage, report address, issues), DKIM per selector, the MX hosts and mail provider, a grade from A to F, a list of fixes, the exact SPF and DMARC records to paste (records_to_paste) and the DKIM steps for the detected mail provider (fix_steps), the source and as_of date, and a page link that shows the same check on the web. It reads public DNS only: it does not send mail, test delivery or read mailboxes.
• check_dns_records, Look up DNS records. Use this when the user asks for a domain's DNS records, its mail provider or which certificate authorities may issue for it, such as "which mail provider does acme.io use?" or "show the MX and CAA records for github.com". Pass the domain and optionally up to 6 record types from A, AAAA, MX, TXT, CAA and NS (default: all). Returns each type's records with TTL, the mail provider named by the MX hosts, and the source and as_of date. It reads public DNS only; it cannot list subdomains or other record types.
• lookup_domain_registration, Look up domain registration. Use this when the user asks who a domain is registered with or when it expires, such as "when does openai.com expire and who is the registrar?". Pass the domain. Returns whether it is registered, the registrar, creation, update and expiry dates, days until expiry, registry status flags, name servers, and the source and as_of date. Registrant and contact details are never returned, so it cannot say who owns a domain.

Add Domain Mail Check in ChatGPT

ChatGPT does not read a local config file. A custom MCP server is added in the product, and only after developer mode is on. These steps were checked against OpenAI's plugin connect guide and the ChatGPT MCP guide on 2026-10-01. Developer mode can be hidden by a workspace policy, so a Team or Enterprise admin may have to allow it first.

1. Open ChatGPT settings, then Security and login, and turn on Developer mode.
2. Open ChatGPT plugins and select the plus button.
3. Enter a name such as "Domain Mail Check" and a one-line description of what you want it to answer.
4. Under Connection, choose the public endpoint and paste this URL. Include the /mcp path. Do not add a tunnel id: this server is already public.

   https://mailcheck.openkrill.app/mcp

5. Create the connection and review the tools ChatGPT lists. You should see launch_score, delivery_check, check_dns_hygiene, check_email_auth, check_dns_records, lookup_domain_registration. The feedback tools on the same server are optional and not required for the lookup.
6. Start a new conversation, add the connection from the tools menu, and ask: Is DMARC set up correctly for github.com?

No API key and no OAuth client id are required. The server answers without a login. Calls are limited per network and per day, and a limit comes back as a tool error rather than a partial answer. If the tool list is stale after a server change, open the connection and select Refresh, then start a new conversation.

The ChatGPT desktop app, Codex CLI and IDE extension share a different config. In the desktop app, open Settings, then MCP servers, choose Add server, select Streamable HTTP, and paste the same URL. In ~/.codex/config.toml the equivalent is a table named mcp_servers.mailcheck with url = "https://mailcheck.openkrill.app/mcp". ChatGPT on the web does not read that file. A published directory listing is also a different path: this page is the developer-mode connection, not a claim that Domain Mail Check is already in the ChatGPT directory.

Treat the server as untrusted until you have read a few answers. A remote MCP server sees the arguments the model sends. Domain Mail Check is read-only and documents what it sends onward on the privacy page.

If it does not work

A call checked on 2026-10-17

github.com publishes SPF and DMARC. The grade is a reading of those records, not a security audit. The request below was posted to https://mailcheck.openkrill.app/mcp as tools/call. HTTP 200. Source: the Domain Mail Check server, read 2026-10-17.

{
  "method": "tools/call",
  "params": {
    "name": "check_email_auth",
    "arguments": {
      "domain": "github.com"
    }
  }
}

• status. ok
• grade. A
• as of. 2026-10-17
• source. Cloudflare DNS over HTTPS (cloudflare-dns.com)
• notice. Checks the exact domain given: SPF and DMARC records of a parent domain are not inherited here. The grade rates protection against spoofing only; it does not read mail or test delivery, and DKIM needs a selector name fro
• dkim status. unknown

Repeat the call yourself if you need a newer reading. Cached answers expire. A rate limit is not a result: wait and try again. Nothing in the call is a ranking, a filing, or advice.

Using it in production

Nothing to install and nothing to run: no package, no Python or Node.js version and no environment variable. Domain Mail Check runs on our servers, and your client only needs the URL https://mailcheck.openkrill.app/mcp.

Use it in other apps

• Domain Mail Check in Claude
• Domain Mail Check in Cursor
• Domain Mail Check in VS Code
• Domain Mail Check in Pi

Roles that use Domain Mail Check

• Startup founder guide

Frequently asked questions

Words on this page

• MCP (Model Context Protocol) definition
• Streamable HTTP transport definition
• Tool primitive definition
• Rate limit policy definition

Source: plugin listing for mailcheck in this repository, and a live tools/call on 2026-10-17. As of 2026-10-17.