Domain Mail Check in Claude

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 Claude

Claude connects to a remote MCP server as a custom connector. The connection is made from Anthropic's cloud, not from your laptop, so the server must be on the public internet. Domain Mail Check is. These steps were checked against Claude's custom-connector article (updated 2026-08-11) and the Claude Code MCP docs on 2026-10-01.

On a Pro or Max plan:

1. Open Customize, then Connectors.
2. Select the plus button, then Add custom connector.
3. Paste this URL. Leave Advanced settings empty: this server does not need an OAuth client id or secret.

   https://mailcheck.openkrill.app/mcp

4. Select Add. The connector is yours. Anthropic has not reviewed it, and it is not a Directory listing.
5. In a conversation, open the plus button, then Connectors, and enable Domain Mail Check. Ask: Is DMARC set up correctly for github.com?

On Team and Enterprise, an Owner adds the connector first at Organization settings, then Connectors, then Add, then Custom, then Web, and pastes the same URL. Members then connect it from Customize. A Free plan can add one custom connector.

In Claude Code, add it from the project directory:

claude mcp add --transport http mailcheck https://mailcheck.openkrill.app/mcp

That writes a local or user config. A JSON entry needs "type": "http" (or streamable-http) plus the url. An entry with a url and no type is skipped. Confirm with claude mcp get mailcheck. The tools you should see are launch_score, delivery_check, check_dns_hygiene, check_email_auth, check_dns_records, lookup_domain_registration.

If Claude tries to fetch https://mailcheck.openkrill.app/mcp from its code sandbox and reports HTTP 403, the block is Claude's sandbox network allowlist, not this server. The sandbox proxy answers 403 with x-deny-reason: host_not_allowed for every host outside the allowlist. Use the connector above instead: connector calls come from Anthropic's cloud and do not pass the sandbox proxy. To let the sandbox reach it as well, an Owner adds mailcheck.openkrill.app under Settings, then Capabilities, then Additional allowed domains.

Enable the connector only in conversations that need it. Claude can call the tools while you work, including during research, so turn off any tool you do not want invoked. Domain Mail Check does not write to your accounts. It still receives the arguments of each call. Read the privacy page before you paste anything that is not public.

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 ChatGPT
• 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.