A static web tool that identifies which provider, security gateway, or forwarder handles a domain's email — derived from public MX records via DNS over HTTPS. Live at https://emailservicechecker.com.
| Category | Examples |
|---|---|
| Mailbox | Google Workspace, Microsoft 365, Zoho, Fastmail, Proton, Migadu, OVH… |
| Consumer | Yahoo, AOL, iCloud, GMX, Yandex, QQ, NetEase 163, Mail.ru, Comcast… |
| Gateway | Proofpoint, Mimecast, Cisco Secure Email, Barracuda, Forcepoint, Sophos… |
| Forwarder | Cloudflare Email Routing, SimpleLogin, addy.io, ForwardEmail.net, Pobox… |
| Inbound API | Amazon SES, Postmark, Mailgun, SendGrid, Mailchimp Transactional… |
Plus three datasets, committed under data/:
willwhite/freemail— 4 459 free consumer mailbox domains.disposable-email-domains/disposable-email-domains— 8 183 disposable / throwaway mailbox providers.data/alias-domains.txt— 18 alias / forwarding domains, hand-curated.
- Bun — package manager, bundler, test runner, dev server.
- TypeScript in strict mode (
tsc --noEmitruns in CI). - Tailwind CSS v4 — tokens in an
@themeblock insrc/styles.css, components in@layer components, animation in CSS. Light and dark schemes (Catppuccin Latte and Mocha). Bricolage Grotesque, Onest and JetBrains Mono are self-hosted via@fontsource-variablepackages and copied intodist/fonts/at build time. No CDN font traffic. - Biome for lint and formatting.
- DNS over HTTPS via
dns.googlefor MX lookups. - GitHub Pages for hosting, deployed by
.github/workflows/deploy.yml.
bun install
bun run dev # builds + serves dist/ at http://localhost:3000The full set of checks, in the order CI runs them:
bun lint # Biome
bun run typecheck # tsc --noEmit
bun test # unit tests over the pure functions
bun run build # writes dist/
bun run verify # checks over the generated outputbun run verify covers what unit tests cannot: that every committed domain
is reachable in the shard the runtime will actually request, that no page
lost its JSON-LD or shipped an unsubstituted {{PLACEHOLDER}}, that every
sitemap URL has a page behind it, and that the domain lists have not crept
back into the bundle.
src/styles.css has two layers, and the distinction matters:
- The palette is Catppuccin as published — Latte, with Mocha overriding
it under
prefers-color-scheme: dark. It is a palette, not a set of decisions. - The semantic tokens (
--color-ink,--color-ink-muted,--color-page,--color-card,--color-field,--color-hairline,--color-accent,--color-accent-text,--color-accent-ink) name what a colour is for. Components and markup use these, never the palette.
Two things depend on that discipline. Catppuccin's overlay-* and
subtext-0 are chrome colours, and using them for text put nine styles
below WCAG AA (subtext-0 measures 4.22:1, overlay-1 2.73:1, against a
4.5:1 requirement) — routing text through --color-ink* fixes them
together rather than one call site at a time. And surfaces used to be
computed by mixing toward a literal white, which cannot invert; naming
them means the dark scheme is a remap of these tokens and nothing else.
Note --color-accent versus --color-accent-text: Latte blue is fine as a
button fill but measures 4.34:1 as link text, so the two roles are separate
tokens. tests/tokens.test.ts enforces all of this — no palette-chrome text
utilities in markup, no hardcoded hex outside the token block, no surface
mixed toward white.
data/free-domains.txt and data/disposable-domains.txt are committed,
so a build needs no network access and is reproducible. bun run datasets
refreshes them from upstream; commit the result.
data/alias-domains.txt is the odd one out: it is hand-curated and never
fetched. There is no maintained public catalogue of alias domains the way
there is for disposable ones, and the set is small enough to keep by hand.
DATASET_SOURCES has no entry for it, which is what keeps bun run datasets
from touching it.
An alias domain is one an alias service hands addresses out on — duck.com,
mozmail.com, privaterelay.appleid.com — so that mail forwards to a real
mailbox the owner keeps private. That is worth separating from the other two:
an alias is not a throwaway that stops existing, and it is not an account
someone logs into. Domains that route mail for a customer's own domain
(ImprovMX, Cloudflare Email Routing) are a different thing again, and are
already matched by MX under the forwarder category in src/providers.ts.
The corporate domains of forwarding companies are deliberately excluded: both
33mail.com and improvmx.com point their apex MX at Google Workspace,
because that is where the company's own staff mail goes. Listing them would
tell someone their perfectly ordinary mailbox was an alias.
They used to be fetched during every deploy, which meant a blip at
raw.githubusercontent.com could break the deploy of an unrelated change,
and upstream edits reached production unreviewed. A scheduled workflow
(.github/workflows/refresh-datasets.yml) now runs the refresh weekly and
opens a PR, so an upstream change arrives as a reviewable diff.
At build time the lists are split into 256 JSON shards under
dist/data/consumer/, and the browser fetches only the shard the queried
domain hashes into — about 800 bytes, in parallel with the DNS lookup.
Inlining them cost 203 kB of a 224 kB main.js; main.js is now 24 kB.
src/consumer-shards.ts holds the hash, imported by both the build and the
runtime so the two cannot disagree about it.
Push to master. The workflow installs Bun, lints, type-checks, tests,
builds, verifies the output, and uploads dist/ to GitHub Pages.
It checks out with fetch-depth: 0 because the sitemap's <lastmod> comes
from each page's last commit date. A shallow clone falls back to the build
date, which is the meaningless signal this replaced.
.
├── src/
│ ├── main.ts – entry, wires form → lookup → render
│ ├── lookup.ts – DoH MX lookup
│ ├── identify.ts – provider matcher + consumer-domain check
│ ├── render.ts – DOM rendering of the report
│ ├── providers.ts – curated MX → provider mapping (161)
│ ├── consumer-shards.ts – shard hash, shared by build and runtime
│ ├── datasets.ts – dataset sources, paths and normalisation
│ ├── types.ts
│ └── styles.css – Tailwind v4 entry: @theme tokens + components
├── data/ – committed domain lists (see above)
├── scripts/
│ ├── build.ts – bundles TS, emits pages, shards, sitemap
│ ├── refresh-datasets.ts – refreshes data/ from upstream
│ ├── verify-build.ts – checks over dist/
│ └── discover-providers.ts – dev helper for finding new providers
├── tests/ – bun test
├── public/ – static assets (logo, CNAME)
├── index.html – template for every generated page
└── .github/workflows/
├── deploy.yml
└── refresh-datasets.yml
Open an issue or PR — especially to suggest new providers or fix
mis-classifications. Each provider is one entry in src/providers.ts.
MIT — see LICENSE.