DEV Community: Ing. Pablo Cueto

How I Built a 4-Layer Email Validator API with FastAPI and Published it on RapidAPI

Ing. Pablo Cueto — Fri, 15 May 2026 22:42:24 +0000

I just launched Email Validator API on RapidAPI — a professional email
verification service built with Python and FastAPI.

Most email validators only check syntax. Mine runs 4 layers of verification
and returns a confidence score for every email.

The 4 Layers

Layer 1 — Syntax Check
Validates format against RFC 5322 standards. Also detects common typos
and suggests corrections — for example user@gmial.com → user@gmail.com.

Layer 2 — DNS / MX Lookup
Confirms the domain exists and is configured to receive emails using
dnspython. Catches domains that look valid but can't receive mail.

Layer 3 — Disposable Domain Detection
Flags throwaway addresses from 3,000+ known providers like Mailinator,
Guerrilla Mail, YOPmail and TempMail. Essential for preventing fake signups.

Layer 4 — SMTP Verification
Performs a real SMTP handshake to confirm the mailbox exists — without
sending a single email. Available on Pro plans.

The Response

Every request returns a detailed JSON breakdown:

{
"email": "user@gmail.com",
"valid": true,
"score": 95,
"syntax_valid": true,
"mx_found": true,
"disposable": false,
"smtp_valid": true,
"suggestion": null,
"error": null
}

Score Interpretation

Score	Meaning
0–29	Invalid syntax
30–54	Domain issues
55–74	Likely valid
75–94	Valid + not disposable
95–100	Fully verified via SMTP

Tech Stack

FastAPI — async endpoints, automatic Swagger docs
dnspython — async DNS/MX lookups
slowapi — rate limiting per plan tier
Render — deployment
RapidAPI — marketplace and monetization

Plans

Plan	Requests/month	Layers	Price
BASIC	100	1–2	Free
PRO	5,000	1–3	$9/mo
MEGA	20,000	1–4	$19/mo
ULTRA	50,000	1–4 + bulk	$29/mo

Try it

🔗 https://rapidapi.com/PabsCueto/api/email-validator-api32

Free plan available — no credit card required.

Happy to answer questions about the implementation!

I Built a Password Strength Analyzer API with Python + FastAPI

Ing. Pablo Cueto — Thu, 14 May 2026 19:00:54 +0000

I Built a Password Strength Analyzer API with Python + FastAPI 🔐

As a backend developer, I wanted to build something useful, real, and monetizable — not just another tutorial project.

So I built a Password Strength Analyzer API that any developer can integrate into their app in minutes.

What it does

Returns a strength score from 0 (very weak) to 4 (very strong)
Estimates crack time using real-world attack scenarios
Detects if the password is in known breach lists
Returns actionable suggestions to improve security
Analyzes uppercase, lowercase, numbers and symbols

Tech Stack

Python 3.10+
FastAPI — for the REST API
zxcvbn — the same algorithm Dropbox uses
Pydantic — data validation
Render — cloud deployment

Example Request

curl -X POST "https://password-analyzern.onrender.com/analyze" \
     -H "Content-Type: application/json" \
     -H "X-API-Key: YOUR_API_KEY" \
     -d '{"password": "MyP@ssw0rd!"}'

Example Response

{
  "password": "***********",
  "score": 3,
  "strength_label": "Fuerte 🟢",
  "crack_time": "3 months",
  "suggestions": ["Idealmente usa 12 o más caracteres"],
  "warnings": [],
  "length": 11,
  "has_uppercase": true,
  "has_lowercase": true,
  "has_numbers": true,
  "has_symbols": true,
  "is_common": false
}

Try it for free

RapidAPI (free plan available):
https://rapidapi.com/PabsCueto/api/password-strength-analyzer1

GitHub:
https://github.com/PabsCueto/password-analyzer

Live docs:
https://password-analyzern.onrender.com/docs

Built this as part of my API portfolio. Feedback welcome!

How I built a PDF Generator API with FastAPI and ReportLab

Ing. Pablo Cueto — Thu, 14 May 2026 18:08:51 +0000

The problem

Almost every SaaS app needs to generate PDFs at some point — invoices,
reports, receipts. Most developers end up wrestling with Puppeteer,
wkhtmltopdf, or heavy libraries that are painful to deploy.

I decided to build a simple REST API that does it with a single POST request.

The stack

FastAPI — for the REST endpoints
ReportLab — for PDF generation in Python
Docker + Render — for deployment
RapidAPI — for monetization

How it works

Send a JSON payload, get a PDF back:

import requests

response = requests.post(
    "https://pdf-generator-api-1fx5.onrender.com/generate/invoice",
    headers={"X-API-Key": "your-key"},
    json={
        "invoice_number": "INV-2025-001",
        "company": {"name": "Acme Corp", "email": "billing@acme.com"},
        "client": {"name": "John Doe", "email": "john@example.com"},
        "items": [
            {"description": "Web Development", "quantity": 10, "unit_price": 150}
        ],
        "tax_rate": 16
    }
)

with open("invoice.pdf", "wb") as f:
    f.write(response.content)

What it generates

Invoice PDFs — with line items, tax calculation and totals
HTML to PDF — convert any HTML string to PDF
Report PDFs — turn tabular data into formatted reports

Try it free

Available on RapidAPI with a free tier (50 requests/month):
🔗 https://rapidapi.com/PabsCueto/api/pdf-generator17

Would love feedback from the community!