DEV Community: Rivier Grullon

Email validation in JavaScript: test your signup rules with mailrules

Rivier Grullon — Tue, 27 Oct 2026 03:00:39 +0000

I was asked to add stronger email validation to our forms at work. Honestly, I thought we were overcomplicating it at first.

Then I started looking into the details.

An email can have valid syntax and still come from a disposable provider. A signup form might need to reject certain domains, restrict access to a company, or flag shared addresses like support@. Someone typing gmial.com probably needs a helpful suggestion.

Those decisions depend on the application. I wanted a way to define them once, inspect the result, and reuse the same policy across projects.

That is what I have been building with mailrules, an MIT-licensed email validation library for JavaScript and TypeScript.

The latest addition is a live playground. You can enter an address, change the rules, see the decision, and copy the configuration into your project before installing anything.

Start with a small signup policy

mailrules uses validator.js for syntax checks and fakeout for its default disposable-domain data.

npm install mailrules

Here is a policy that rejects known disposable domains and a custom blocked domain, suggests possible typos, and warns about role addresses:

import { createEmailValidator } from 'mailrules';

const validator = createEmailValidator({
  blocklist: { domains: ['spam.example'] },
  suggestions: true,
  roleAccounts: 'warn',
});

validator.check('alice@example.com').valid;
// true

validator.check('hello@mailinator.com').reasons;
// ['disposable']

validator.check('person@spam.example').reasons;
// ['blocked']

validator.check('support@example.com').warnings?.[0]?.code;
// 'role_account' — a warning; the address is still accepted

validator.check('alex@gmial.com').suggestion;
// 'alex@gmail.com'

Suggestions are advisory. They never replace the submitted address or change whether it passes validation. A result can contain both a rejection and a suggestion; for example, the installed disposable dataset also flags gmial.com. If someone chooses the suggested address, validate that new value too.

See why an address was rejected

A boolean is useful for a form, but it does not explain which configuration entry caused a rejection. For that, there is explain():

const result = validator.explain('person@sub.spam.example');

result.reason;
// 'blocked'

result.matchedRule;
// {
//   source: 'blocklist',
//   type: 'domain',
//   value: 'spam.example',
//   matchedSubdomain: true
// }

Domain rules include subdomains by default. You can turn that off with includeSubdomains: false.

Allowlists also need predictable behavior: an explicit block always wins in mailrules. An allowlist can exempt an address from disposable rejection, but it does not override a blocklist entry. For an invitation-only or company-only form, allowlistMode: 'only' requires an allowlist match.

Try the policy before wiring it into a form

In the playground, you can edit domain and mailbox lists, switch role-address and free-provider policies, inspect the result, and copy JavaScript or React examples.

Role addresses and free providers each support classification, warnings, or blocking. Both checks are off by default. A provider-list non-match does not prove an address belongs to a business, and a Gmail address is not inherently a bad signup.

The playground also has a working React Hook Form + Zod example, so you can see how a policy affects field errors and normalized output. Validation happens locally in your browser; the playground does not send the entered address to a validation service.

Reuse it with Zod and React Hook Form

The optional Zod 4 adapter turns the same validator into a schema:

npm install mailrules zod

import { z } from 'zod';
import { zodEmail } from 'mailrules/zod';

// Reuse the validator from the first example.
const signupSchema = z.object({
  email: zodEmail(validator),
});

signupSchema.parse({ email: ' Alice@EXAMPLE.COM ' });
// { email: 'Alice@example.com' }

signupSchema.safeParse({ email: 'hello@mailinator.com' }).success;
// false

You can pass that schema to React Hook Form through zodResolver. The integration guide includes the full form example and a Standard Schema adapter for tools that support it.

Warnings remain successful schema results. Use check() or explain() when you want to display those warnings separately. Apply the policy on the backend accepting submissions as well, since browser checks can be bypassed.

React, Zod, and the playground are not required downloads for a normal mailrules installation. Install Zod when you use its adapter. The playground lives in the same repository but is excluded from the npm package.

There is also a CSV command

For reviewing an existing list with an email column:

npx mailrules --input contacts.csv --status rejected --output rejected.csv

This exports rejected rows with validation reasons. You can also export accepted rows or supply a JSON rules configuration. The CLI runs on Node.js 22.12 or newer and processes bounded files in memory; the guide documents its limits.

What “accepted” means here

mailrules checks syntax and your configured application rules locally, in a browser bundle or Node.js. It does not check DNS, MX records, or SMTP, and an accepted result does not establish that a mailbox exists or that the person submitting it owns it.

There are tools with a broader verification scope, including email-validator-js, which documents MX and SMTP checks and a hosted API. For this project, I wanted reusable local policies with clear explanations.

If you have dealt with messy signup forms, I would love to hear about a rule that was harder to get right than you expected. Especially cases where a strict rule ended up rejecting legitimate users.

Email validation with disposable detection, blocklists and allowlists in TypeScript

Rivier Grullon — Tue, 27 Oct 2026 00:25:01 +0000

An email can have valid syntax and still fail your application's signup rules. You might want to reject disposable providers, block a specific mailbox, or accept only addresses from a company's domain.

I built mailrules to bring those decisions into one configurable TypeScript/JavaScript API. Version 0.1.0 is available on npm.

Start with a reusable validator

npm install mailrules

import { createEmailValidator } from 'mailrules';

const validator = createEmailValidator({
  blocklist: {
    domains: ['spam.example'],
    emails: ['abuse@example.com'],
  },
  allowlist: {
    emails: ['trusted@mailinator.com'],
  },
});

validator.check('person@mailinator.com').reasons; // ['disposable']
validator.check('person@spam.example').reasons; // ['blocked']
validator.check('trusted@mailinator.com').valid; // true
validator.check(null).reasons; // ['invalid_format']

Create the validator once and reuse it. Lists are copied, normalized and indexed during construction. Validation is synchronous and makes no network requests.

The default entry uses validator.js for syntax and fakeout for disposable domains. The underlying dataset comes from the disposable-email-domains project.

Make the policy explicit

An allowlist entry normally creates an exception to disposable-email rejection. To restrict access to specific domains or addresses, use allowlistMode: 'only':

const teamOnly = createEmailValidator({
  allowlistMode: 'only',
  allowlist: {
    domains: ['company.example'],
    emails: ['contractor@partner.example'],
  },
});

teamOnly.check('staff@company.example').valid; // true
teamOnly.check('staff@eu.company.example').valid; // true
teamOnly.check('someone@gmail.com').reasons; // ['not_allowed']

Domain rules include subdomains by default. Set includeSubdomains: false to require exact domain matches.

Rejection checks run in a defined order:

Invalid email syntax.
Explicit blocklist match.
Missing allowlist match in 'only' mode.
Disposable domain without an allowed exception.

An explicit block always wins over an allowlist exception. The result includes a stable reason code so the application can decide what feedback to show.

Bring your own disposable-domain data

Use the core entry when you want to manage that data yourself:

import { createEmailValidator } from 'mailrules/core';

const customValidator = createEmailValidator({
  disposableDomains: ['burner.example', 'temporary.example'],
});

customValidator.check('user@burner.example').reasons; // ['disposable']

mailrules/core defaults to an empty disposable dataset and does not import fakeout. The npm package still declares fakeout as a dependency. A supplied dataset replaces the default dataset.

Installed data does not refresh automatically. Update dependencies and redeploy to pick up changes, or supply updated data and construct a new validator.

What the result tells you

valid: true means the address passed syntax checks and your configured rules. It does not establish that the mailbox exists or belongs to the person submitting it. There are no DNS, MX or SMTP checks; ownership still requires a confirmation flow.

Domain names are normalized, while local-part case and plus tags are preserved. Exact mailbox rules therefore distinguish User@example.com from user@example.com.

The package supports ESM, CommonJS, Node.js 22.12+ and modern browser bundlers. Apply the same policy on the server when accepting form submissions.

I'd appreciate feedback on the rule precedence and API, especially from developers who have had to handle exceptions to disposable-email blocking.

Source and documentation · npm package

Rate Forge: Calculate the Exact Hourly Rate You Need as a Freelancer

Rivier Grullon — Thu, 05 Mar 2026 22:24:40 +0000

Calculate your real minimum freelance rate — self-employment taxes, healthcare, overhead, and utilization all factored in.

🔗 Try it free: https://microsaas-lab.vercel.app/rate-forge

The Problem

Most freelancers set rates by looking at what others charge and picking a competitive-sounding number. This is how you end up working 50 hours a week but taking home far less than expected.

Freelance rates need to account for things employed workers never think about: self-employment taxes, healthcare, non-billable time.

Introducing Rate Forge

Rate Forge walks you through every cost factor: target take-home salary, self-employment taxes, healthcare, retirement contributions, overhead, vacation days, and realistic billable utilization. The output is the minimum you must charge.

Rate Forge is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Target salary with take-home reverse calculation
Self-employment tax calculator
Healthcare premium input
Retirement contribution planning
Overhead costs (software, equipment, professional development)
Billable utilization rate — accounts for non-billable time
Minimum hourly rate with buffer recommendations

Who Is It For?

Freelancers of all types — developers, designers, writers, consultants — who want to set rates that actually cover their real costs.

Try It Free

👉 https://microsaas-lab.vercel.app/rate-forge

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Clean My Leads: Clean, Normalize, and Deduplicate Messy CRM Lead Lists in One Click

Rivier Grullon — Thu, 05 Mar 2026 22:22:52 +0000

Clean, normalize, and deduplicate messy lead lists and CRM exports in one click. Import-ready CSV in seconds.

🔗 Try it free: https://microsaas-lab.vercel.app/clean-my-leads

The Problem

Every sales team eventually ends up with a lead list disaster. Names in ALL CAPS mixed with lowercase, phone numbers in 6 different formats, emails with trailing spaces, duplicates everywhere.

CRM imports that fail validation are one of the most frustrating sources of wasted time in sales operations.

Introducing Clean My Leads

Clean My Leads applies intelligent normalization in one pass: standardizes phone formats, title-cases names, strips whitespace, deduplicates on email or phone, and produces an import-ready CSV for any CRM.

Clean My Leads is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Normalizes phone numbers to E.164 or local format
Standardizes email addresses (lowercase, trim whitespace)
Applies proper name casing (Title Case)
Deduplicates by email and/or phone number
Flags missing required fields for manual review
Exports clean import-ready CSV — works with any CRM

Who Is It For?

Sales reps, sales ops teams, SDRs, and marketers working with lead lists from multiple sources.

Try It Free

👉 https://microsaas-lab.vercel.app/clean-my-leads

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Thread2Print: Convert Twitter/X Threads into Clean, Printable Documents

Rivier Grullon — Thu, 05 Mar 2026 22:20:37 +0000

Paste any Twitter/X thread or wall of text and get a clean, distraction-free, printable version instantly.

🔗 Try it free: https://microsaas-lab.vercel.app/ThreadPrint

The Problem

Twitter/X threads contain some of the best expert knowledge on the internet — but reading them on the platform means fighting infinite scroll, retweet noise, ads, and trending topics.

Some of the best writing is trapped in thread format.

Introducing Thread2Print

Thread2Print converts any thread (or any wall of text) into a clean, readable, printable document. Paste the content, get back a beautifully formatted page you can read, print, or save as PDF — stripped of all platform noise.

Thread2Print is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Paste any thread or text — auto-detects and formats
Clean typography optimized for long-form reading
Print-ready layout — save as PDF from browser
Strips retweet noise and platform clutter
Preserves links and media captions
100% browser-based, instant

Who Is It For?

Thread readers, knowledge workers, students, and anyone who wants to save threads for offline reading.

Try It Free

👉 https://microsaas-lab.vercel.app/ThreadPrint

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Timestamp Ninja: Generate YouTube Chapter Timestamps and Podcast Show Notes Instantly

Rivier Grullon — Thu, 05 Mar 2026 22:20:03 +0000

Generate YouTube chapter timestamps and podcast show notes from your recording notes in seconds. Stop manual time-coding.

🔗 Try it free: https://microsaas-lab.vercel.app/TimeStampNinja

The Problem

After finishing a 2-hour recording, you have to go back and manually write timestamps for every section. Scrub through the video, note the times, format them into YouTube chapter format, do it again for podcast show notes.

This can take 30-60 minutes per episode, every single time.

Introducing Timestamp Ninja

Timestamp Ninja takes your raw recording notes — even rough ones with approximate times — and formats them into publication-ready YouTube chapters and podcast show notes in one pass.

Timestamp Ninja is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Parse rough time notes and auto-format to HH:MM:SS or MM:SS
Generate YouTube chapter descriptions (correct format)
Generate podcast show notes (multiple formats supported)
Batch processing — handle a full episode in one paste
Export as plain text, Markdown, or formatted HTML
100% browser-based, instant

Who Is It For?

YouTubers, podcasters, content creators, and video editors who produce long-form audio or video.

Try It Free

👉 https://microsaas-lab.vercel.app/TimeStampNinja

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

BatchResize: Resize Images to Every Social Media Format in One Shot

Rivier Grullon — Thu, 05 Mar 2026 22:19:36 +0000

Resize your images to every social media format in one shot — Instagram, Twitter/X, LinkedIn, TikTok, and more. Zero uploads.

🔗 Try it free: https://microsaas-lab.vercel.app/batch-resizer

The Problem

Social media has fragmented image dimensions into a nightmare. Instagram wants 1:1, 4:5, and 16:9. Twitter wants 16:9. LinkedIn has its own sizes. Every piece of content requires resizing the same image 6-8 times.

For solo creators or small marketing teams, this is hours of repetitive work every week.

Introducing BatchResize

BatchResize processes all your social media formats in one pass. Upload your image, select your target platforms, and download all resized versions in a ZIP — processed entirely in your browser with no upload required.

BatchResize is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Covers Instagram (Feed, Story, Reel), Twitter/X, LinkedIn, TikTok, Facebook, YouTube
Smart crop modes: center, face-aware, manual
Batch all platforms in one click
Downloads as ZIP with properly named files
Zero cloud uploads — processed in your browser
No watermarks, no account required

Who Is It For?

Social media managers, content creators, marketing teams, and solo founders posting across multiple platforms.

Try It Free

👉 https://microsaas-lab.vercel.app/batch-resizer

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

WebP Cruncher: Batch Convert Images to WebP for Faster Page Loads and Better SEO

Rivier Grullon — Thu, 05 Mar 2026 22:18:36 +0000

Batch convert and compress images to WebP for faster page loads and better SEO scores. No uploads, no limits, no watermarks.

🔗 Try it free: https://microsaas-lab.vercel.app/image-cruncher

The Problem

PageSpeed Insights keeps telling you to serve images in next-gen formats. WebP delivers 25-35% smaller files than JPEG or PNG. But converting your existing image library is tedious.

Most tools process server-side (a privacy concern), watermark the output, or limit batch size behind a paywall.

Introducing WebP Cruncher

WebP Cruncher converts your entire image library to WebP in a single batch, using the Canvas API directly in your browser. No file size limits, no watermarks, no uploads — drag in your images and download a ZIP.

WebP Cruncher is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Batch convert any number of images to WebP
Quality slider (0-100) with live file size preview
Supports PNG, JPEG, GIF (first frame), and BMP input
Downloads all converted images as a ZIP
100% browser-based — no server uploads
No file size limits, no watermarks, completely free

Who Is It For?

Web developers, SEO professionals, site owners, and anyone improving page load speed.

Try It Free

👉 https://microsaas-lab.vercel.app/image-cruncher

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

POC Nexus Pro: Render HTML Proof-of-Concepts in Mobile, Tablet, and Desktop Device Frames

Rivier Grullon — Thu, 05 Mar 2026 22:18:08 +0000

Render and screenshot any HTML POC inside mobile, tablet, or desktop device frames. Perfect for QA reports and client demos.

🔗 Try it free: https://microsaas-lab.vercel.app/pocViewer

The Problem

Presenting an HTML proof-of-concept to clients means either sending a raw HTML file, taking janky screenshots, or spinning up a hosted demo for a 5-minute review.

None of these are professional.

Introducing POC Nexus Pro

POC Nexus Pro renders your HTML file inside realistic device frames — iPhone, iPad, MacBook, desktop monitor — and lets you take clean screenshots without needing real devices.

POC Nexus Pro is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Renders HTML files inside mobile, tablet, and desktop device frames
Multiple device choices: iPhone, iPad, Android, MacBook, desktop
Screenshot export at multiple resolutions
Responsive preview — test how your layout adapts
Multiple frame themes (light/dark bezels)
100% browser-based

Who Is It For?

Frontend developers, UI/UX designers, QA engineers, and freelancers presenting work to clients.

Try It Free

👉 https://microsaas-lab.vercel.app/pocViewer

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Bulk Redirect Validator: Validate Hundreds of URL Redirects in Seconds

Rivier Grullon — Thu, 05 Mar 2026 22:16:45 +0000

Validate hundreds of URL redirects by generating JS console scripts or cURL bash commands — no server access needed.

🔗 Try it free: https://microsaas-lab.vercel.app/redirect

The Problem

After a site migration you need to verify hundreds of redirects are working correctly. Doing this manually is impossible at scale, and most tools require uploading your URL list to a third-party service.

There's a better way that keeps your data private.

Introducing Bulk Redirect Validator

Bulk Redirect Validator generates JavaScript console scripts and cURL bash commands you run yourself to test redirect chains. Paste your URL pairs, get a test script, run it — no third-party server sees your URLs.

Bulk Redirect Validator is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Input source and destination URL pairs in bulk
Generate JavaScript console test script (run in Chrome DevTools)
Generate cURL bash script (run in terminal)
Test for 301, 302, 307, 308 and final destination
Detect redirect chains and loops
Export results as CSV for client reporting

Who Is It For?

SEO professionals, web developers, digital agencies, and site admins managing URL migrations.

Try It Free

👉 https://microsaas-lab.vercel.app/redirect

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Scope Forge: Build Professional Freelance Estimates That Actually Close Deals

Rivier Grullon — Thu, 05 Mar 2026 22:15:47 +0000

Build professional project estimates for freelance clients in minutes. Break down scope, set rates, export a polished PDF quote.

🔗 Try it free: https://microsaas-lab.vercel.app/scoopeForge

The Problem

Freelancers often lose deals not because of price, but because of how they present it. A Google Doc with bullet points and a total number doesn't convey expertise or value.

Building a detailed, professionally formatted proposal from scratch for every client is time-consuming and inconsistent.

Introducing Scope Forge

Scope Forge is a structured estimate builder: scope breakdown, deliverables, timeline, rates, and terms. The output is a polished PDF proposal that looks like it came from an agency — not a Google Doc.

Scope Forge is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Structured scope builder — deliverables, phases, and milestones
Hourly or fixed-price estimate modes
Time estimate breakdown per deliverable
Payment terms and deposit configuration
Professional PDF export with your branding
Save and reuse estimate templates

Who Is It For?

Freelancers, consultants, and small agencies who want to present more professional estimates and reduce price negotiation.

Try It Free

👉 https://microsaas-lab.vercel.app/scoopeForge

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.

Sheet2Mock: Convert Excel/Google Sheets Data to Clean JSON + TypeScript Interfaces

Rivier Grullon — Thu, 05 Mar 2026 22:14:57 +0000

Paste your spreadsheet data and instantly get clean JSON mock data plus TypeScript interfaces. Zero config, 100% in-browser.

🔗 Try it free: https://microsaas-lab.vercel.app/sheet-to-json

The Problem

Frontend developers often wait weeks for real API data before they can build and test UIs. The workaround is writing mock data by hand — creating JSON arrays, TypeScript interfaces, keeping both in sync as the schema evolves.

This is repetitive, error-prone, and boring.

Introducing Sheet2Mock

Sheet2Mock converts your spreadsheet data (pasted from Excel or Google Sheets) into a JSON array and TypeScript interface definition in one step. Paste your data with headers, get production-ready mocks to drop directly into your codebase.

Sheet2Mock is completely free, requires no account or installation, and runs entirely in your browser. Your data never leaves your machine.

Key Features

Paste CSV or tabular data from Excel or Google Sheets
Generates clean JSON array with inferred data types
Generates TypeScript interface matching the schema
Type inference: detects strings, numbers, booleans, dates
One-click copy for JSON and TypeScript separately
100% in-browser — no API calls, no uploads

Who Is It For?

Frontend developers who need mock data before the backend is ready, and anyone who receives data requirements in spreadsheet format.

Try It Free

👉 https://microsaas-lab.vercel.app/sheet-to-json

No sign-up. No install. Open it and start immediately.

If you find it useful, share it with someone who could benefit. I'm building more free tools at The Lab — explore the whole collection.

Have feedback or a feature request? Drop a comment below — I read everything.