Email Validator API

Live instance: https://email.lifestep.io — try /health, and see /openapi.json for the full spec. Free, no signup.

Self-host it: docker compose up -d (see below). MIT licensed.

A small FastAPI utility for checking email-address syntax, mail-routing DNS, and common risk signals without sending an email and without a paid third-party API.

Run locally

python3.12 -m venv .venv
.venv/bin/pip install -r requirements.txt
.venv/bin/uvicorn app.main:app --host 127.0.0.1 --port 8000

Or run the hardened container definition:

docker compose up --build

Docker Compose maps the service to http://127.0.0.1:8087. Interactive API docs are at /docs; OpenAPI JSON is always available at /openapi.json.

Endpoints

• GET /validate?email=person@example.com
• POST /validate with {"email":"person@example.com"}
• POST /validate with {"emails":["one@example.com","two@example.com"]} (1–50 addresses)
• GET /health
• GET / for a tiny usage page

Examples:

curl --get http://127.0.0.1:8087/validate \
  --data-urlencode 'email=User@BÜCHER.de'

curl http://127.0.0.1:8087/validate \
  -H 'Content-Type: application/json' \
  -d '{"emails":["person@gmail.com","person@gmial.com"]}'

A single request returns one result object. A batch returns {"count": 2, "results": [...]}. Each result contains:

{
  "input": " person@GMIAL.com ",
  "normalized": "person@gmial.com",
  "syntax_valid": true,
  "domain": "gmial.com",
  "is_ascii": true,
  "idn": false,
  "has_mx": false,
  "mx_hosts": [],
  "is_disposable": false,
  "is_role_account": false,
  "is_free_provider": false,
  "suggestion": "person@gmail.com",
  "score": 0,
  "verdict": "undeliverable",
  "note": "No MX, A, or AAAA mail-routing records were found."
}

normalized trims surrounding whitespace, preserves the local-part's case, and lowercases the domain. Unicode domains are IDNA/punycode encoded; idn reports that conversion. is_ascii reports whether the trimmed input was entirely ASCII.

Checks and scoring

• Practical RFC-5322-style dot-atom syntax, including local-part length ≤64 and total normalized length ≤254. This intentionally does not accept every obscure quoted-address form allowed by the full RFC.
• MX lookup with a 3-second lifetime. When there is no MX response, A and AAAA are checked as the RFC 5321 implicit routing fallback. Up to three MX hosts are returned in preference order.
• DNS results are cached in process for 600 seconds. Each worker has its own cache; it is intentionally not shared or persistent.
• Batch validation uses a bounded 10-thread pool so independent DNS waits do not serialize all 50 addresses.
• A static bundled set of 218 established disposable-mail domains, plus role-account and common free-provider sets. Nothing is fetched at runtime.
• Provider typo suggestions use Levenshtein edit distance. A suggestion is the full corrected address, not an automatic rewrite.

The score begins at 60 for valid syntax, adds 30 for usable mail-routing DNS, and subtracts risk weights for missing/unknown DNS, disposable domains, role accounts, or a likely provider typo. It is clamped to 0–100. Scores ≥75 are deliverable_likely, 40–74 are risky, and lower scores are undeliverable.

Honest limits

• No SMTP probe is performed. The API never connects to a mail server or sends mail.
• Syntax and DNS cannot prove that a mailbox exists, accepts mail, is monitored, or will accept a particular future message. There is no deliverability guarantee.
• has_mx: true means usable mail-routing DNS was found. It can come from an MX record or, when MX is absent, the RFC 5321 A/AAAA fallback; mx_hosts remains empty for an address-record fallback.
• has_mx: null means DNS was unavailable or timed out, not that the address is invalid. The note field explains the state.
• Disposable-domain lists, role labels, free-provider lists, typo detection, score, and verdict are heuristics. Domains and provider behavior change over time.
• Internationalized domains are supported through IDNA. Unicode local parts (SMTPUTF8) are not accepted by this practical validator.

Tests

python3.12 -m venv .venv
.venv/bin/pip install -r requirements-dev.txt
PYTHONPATH=. .venv/bin/pytest -q

The test suite mocks DNS; it does not depend on public resolvers or network access.

Related free tools

• CLAUDE.md Auditor - paste your rules file and see which rules an agent cannot reliably follow
• XLSX Inspector — check workbooks for macros, external links and hidden sheets
• DNS and SPF Check — records, SPF, DMARC and TLS expiry
• Email Validator — syntax, MX, disposable and role addresses
• QR Code Generator — free PNG and SVG API, no signup
• agent-watch
• ai-code-review-prompts
• claude-md-patterns
• claude-code-orchestration-ko
• xlsx-inspector-api
• domain-info-api
• qr-code-api
• Agent Ops for VS Code - review prompts and agent rules in the Command Palette (VSIX install)
• Go Exec Format Doctor Action - CI gate for binary architecture mismatches

The paid guide collection is available at lifestep1.gumroad.com.