17k+

Disposable domains

Throwaway blocklist, refreshed every 24h

Validation checks

Format, MX, SMTP, catch-all, disposable, role

0–1

Confidence score

Weighted, clamped, adjustable threshold

REST

No SDK required

One GET request, JSON response

Hosted API · No infra required

Email validation
with real SMTP
proof.

One GET request returns format validity, MX records, a live SMTP probe, catch-all detection, disposable blocking, and a 0–1 confidence score. Sign up, grab a key, start validating in minutes.

Get started free API reference

Auth method

X-Api-Key header

SMTP probe

RCPT TO · port 25

Greylist retry

×2 · 2s delay

Fast mode

smtp=false · <100ms

Billing

Stripe · monthly quota

Free tier

100 lookups / month

API response example · GET /[email protected]

$ curl -s -H "X-Api-Key: lj_live_xKj4mN" \ "https://api.letterjudge.com/[email protected]" { "email": "[email protected]", "format_valid": true, "mx_found": true, "mx_records": ["mail.acme.io"], "smtp_reachable": true, "smtp_detail": "RCPT accepted", "catch_all": false, "disposable": false, "role_address": false, "score": 0.9, "did_you_mean": null }

FEATURES Six validation layers — nothing to configure

Format validation

Instant format check

Regex check on email structure. Fails fast before any I/O. Returns score 0.0 immediately on bad format.

RFC 5321 · max 254 chars

DNS resolution

MX record lookup

DNS resolution with 5s timeout. Returns all records sorted by priority. No MX = score stays at 0.2.

dnspython · 5s timeout

SMTP probe

Real mailbox verification

Real RCPT TO connection to the mail server. Handles greylisting with 2 automatic retries before reporting inconclusive.

RCPT TO · port 25 · retry ×2

Catch-all detection

Expose false positives

Second probe with a nonsense address exposes domains that accept any RCPT TO. A 250 response doesn't mean the mailbox exists.

zzz-no-such-mailbox probe

Disposable blocking

17,000+ blocked domains

Throwaway domain blocklist from the open-source disposable-email-domains registry. Auto-refreshed every 24 hours.

GitHub blocklist · 24h refresh

Role address flag

Identify team inboxes

Identifies generic inboxes (admin@, info@, noreply@, support@) that belong to teams, not people. Penalised −0.10.

admin, info, noreply, support…

PIPELINE Sequential validation — each step runs only if the previous passes

Step 01 · Format

Format check

Regex validates email structure before touching the network. Returns immediately with score 0.0 on failure.

RFC 5321 · max 254 chars · microseconds

Step 02 · MX Lookup

DNS resolution

DNS resolves MX records for the domain. Records returned sorted by priority with a 5s timeout.

dnspython · 5s timeout · sorted by priority

Step 03 · SMTP Ping

RCPT TO probe

Connects to the top-priority MX and issues a RCPT TO probe. 4xx greylisting retried twice before marking inconclusive.

RCPT TO · port 25 · greylist retry ×2

Step 04 · Catch-All

False positive check

A second probe to a guaranteed-nonexistent address detects domains that accept everything. Score penalised −0.10.

zzz-no-such-mailbox@domain · −0.10 penalty

API One endpoint · stateless · no SDK required

Endpoint & parameters

GET /validate

Parameter	Type		Description
email	string	required	Address to validate. Max 254 chars.
X-Api-Key	header	required	Your API key (lj_live_…).
smtp	boolean	optional	Run SMTP probe. Default true. Set false for fast MX-only.

# full check
curl "https://api.letterjudge.com/[email protected]" \
  -H "X-Api-Key: lj_live_xKj4mN"

# skip SMTP for speed
curl "https://api.letterjudge.com/[email protected]&smtp=false" \
  -H "X-Api-Key: lj_live_xKj4mN"
import requests

r = requests.get(
    "https://api.letterjudge.com/validate",
    headers={"X-Api-Key": "lj_live_xKj4mN"},
    params={"email": "[email protected]"},
    timeout=20,
)
r.raise_for_status()
data = r.json()

if data["score"] >= 0.6:
    print("looks good")
// Laravel / vanilla PHP
$response = Http::withHeaders([
    'X-Api-Key' => 'lj_live_xKj4mN',
])->get('https://api.letterjudge.com/validate', [
    'email' => '[email protected]',
]);

$data = $response->json();
return $data['score'] >= 0.4
    && !$data['disposable']
    && $data['mx_found'];
const res = await fetch(
  `https://api.letterjudge.com/[email protected]`,
  { headers: { "X-Api-Key": "lj_live_xKj4mN" } }
);
const data = await res.json();

const valid = data.score >= 0.6
  && !data.disposable
  && data.mx_found;

Response schema

emailstringNormalised address as submitted.

format_validboolRegex check on email structure.

mx_foundboolDomain has at least one valid MX record.

mx_recordsstring[]MX hostnames sorted by priority.

smtp_reachablebool | nulltrue = accepted · false = rejected · null = inconclusive.

smtp_detailstring | nullHuman-readable SMTP outcome or error.

catch_allbool | nullDomain accepts mail to any address. Score −0.10.

disposableboolFound in 17k-entry throwaway blocklist.

free_providerboolGmail, Outlook, Yahoo, iCloud, etc.

role_addressboolGeneric prefix: admin, info, noreply…

scorefloatWeighted confidence 0.0 – 1.0.

did_you_meanstring | nullCorrected address for common typos.

Error responses

401Invalid or missing API key.

429Monthly quota exceeded. Upgrade to continue.

422Email exceeds 254 characters.

SCORING Weighted confidence score 0.0 – 1.0 · set your own acceptance threshold

Factor

Weight

Valid format Regex passed

+0.20

MX record found Domain has mail servers

+0.30

SMTP accepted 250 OK from server

+0.50

SMTP inconclusive MX found, no SMTP answer

+0.25

SMTP rejected 5xx permanent error

−0.10

Catch-all domain Accepts any RCPT TO

−0.10

Disposable domain Throwaway provider

−0.20

Role address admin@, info@, noreply@…

−0.10

Suggested thresholds

≥ 0.4Lenient

format + MX only

≥ 0.6Balanced

MX + inconclusive SMTP

≥ 0.7Strict

confirmed SMTP + not catch-all

≥ 0.9Very strict

SMTP + clean + not role

SMTP caveats

SMTP probing runs from our infrastructure. Pass smtp=false for sub-100ms format + MX checks.

Greylisting causes temporary 4xx — retried twice with a 2s delay before returning smtp_reachable: null.

PRICING Start free · upgrade via Stripe billing portal · cancel any time

Free

$0 /mo

100 lookups / month

Format validation
MX lookup
Disposable detection
Role address flag
SMTP probe
Catch-all detection

Get started

Starter Popular

$15 /mo

5,000 lookups / month

Everything in Free
SMTP probe
Catch-all detection
Greylisting retry
Usage dashboard
Key rotation

Start free trial

Pro

$29 /mo

50,000 lookups / month

Everything in Starter
Multiple API keys

Get Pro

Business

$99 /mo

Unlimited lookups

Everything in Pro
Unlimited validations
Priority email support

Get Business

QUICKSTART Validating emails in minutes — no servers, no config files

Create a free account

Sign up at letterjudge.com. Your API key is generated immediately and shown once in the onboarding screen. No credit card required on the free tier.

dashboard → onboarding

Your API key lj_live_xKj4mN8pQr3vL7wEs... Copy it now — it won't be shown in full again.

Copy your API key

Your key appears once at signup and is always visible in the dashboard. It starts with lj_live_. Store it securely — treat it like a password.

dashboard → API Keys

Your key lj_live_xKj4mNxxxxxxxx... Created May 23, 2026 · Last used just now

Make your first request

Pass the key in the X-Api-Key header. Add smtp=false for sub-100ms MX-only checks when latency matters.

bash

$ curl -s "https://api.letterjudge.com/[email protected]" \ -H "X-Api-Key: lj_live_xKj4mN" | jq .score 0.9

Free tier: 100 lookups/month, no card required. SMTP probing included on all paid plans. Pass smtp=false for faster format + MX-only checks.

complete walkthrough

1 · full validation (SMTP included)

$ curl -s "https://api.letterjudge.com/[email protected]" \ -H "X-Api-Key: lj_live_xKj4mN8pQr3vL7wEs..." | jq '{score,smtp_reachable,disposable}' { "score": 0.9, "smtp_reachable": true, "disposable": false }

2 · fast check (MX only, ~50ms)

$ curl -s "https://api.letterjudge.com/[email protected]&smtp=false" \ -H "X-Api-Key: lj_live_xKj4mN8pQr3vL7wEs..." | jq .score 0.5

3 · check usage

$ curl -s https://api.letterjudge.com/usage \ -H "X-Api-Key: lj_live_xKj4mN8pQr3vL7wEs..." | jq '{plan,requests_this_period,limit}' { "plan": "free", "requests_this_period": 3, "limit": 100 }

Email validationwith real SMTPproof.

Instant format check

MX record lookup

Real mailbox verification

Expose false positives

17,000+ blocked domains

Identify team inboxes

Format check

DNS resolution

RCPT TO probe

False positive check

Suggested thresholds

Create a free account

Copy your API key

Make your first request

Email validation
with real SMTP
proof.