How do you verify 100,000 emails without waiting hours?

You’re running a campaign. Your list is ready. You hit send—then wait. Hours. No progress. The system locks up, and your pipeline stalls because the verification step is synchronous: one email at a time, blocking everything until the last one finishes.

That’s not just slow. It’s wasteful. Your app doesn’t need to stall. With async bulk validation with webhooks and callbacks, you submit the list, continue your workflow, and get results when they’re ready—no waiting.

Imagine sending a batch of 100,000 emails to your verifier not as a stopgap, but as a fire-and-forget job. While it runs in the background, your system processes more data, triggers follow-ups, or prepares for the next step—all without interruption.

Key takeaways

  • Async bulk validation with webhooks and callbacks eliminates pipeline blockers by allowing your app to continue before verification completes.
  • Webhooks deliver results as they’re ready, not all at once—ideal for high-volume processing without memory strain.
  • Unlike synchronous checks, this method doesn't require polling or idle waits; it integrates smoothly into automated workflows.

What is async bulk validation with webhooks and callbacks?

Async bulk validation means you submit a large list of emails and don’t wait for results in real time—you get notified when processing finishes. Webhooks are automated HTTP POSTs sent to your server when verification completes, delivering results directly. Callbacks let you trigger downstream actions—like updating a CRM or logging data—without polling, making your system efficient and reactive.

Why async processing beats waiting

When you submit thousands of emails, waiting for each result to come back synchronously slows down your entire workflow. Async processing lets you hand off the task and move on. You’re no longer blocked, and your API doesn’t get overwhelmed by long-running requests.

Think of it like sending a batch of letters. Instead of standing by the mailbox, you’re told when a reply arrives—no need to check every five seconds. This is how high-throughput systems scale. RFC 5321, the core SMTP specification, handles such asynchronous behavior naturally through message queuing in mail servers, confirming that this isn't just a backend trick—it's built into email infrastructure.

How webhooks and callbacks work together

Once your list finishes validating, we send a webhook to the endpoint you provide. This is a secure HTTP request with a JSON payload containing the result for each email. You don’t have to check back—your system receives the data as soon as it’s ready.

Use cases are straightforward: update your user database, trigger a notification in Slack, or push clean emails to your CRM. For example, if you're using HubSpot or SendGrid, you can integrate seamlessly with our real-time verification API. You can even set up automated flows in tools like Zapier using the callback payload.

Webhooks eliminate polling—the constant, inefficient checks that waste bandwidth and API calls. Instead, you’re notified only when something changes. This is standard in production systems, and platforms like AWS Lambda or Cloudflare Workers are built to handle these events efficiently.

To see how this fits into your workflow, check our bulk email list cleaning solution, designed for teams sending at scale. You can submit a file, set up a webhook endpoint, and let the validation run while you focus on the next task. The results arrive exactly when you’re ready to act.

Why async validation is essential for large-scale list hygiene

You can’t clean 100,000 email addresses with real-time checks without freezing your system. Async validation with webhooks and callbacks lets you process massive lists without blocking workflows, enabling seamless integration with CRMs, data warehouses, and automated campaigns—no matter how big your list grows.

Scale demands more than real-time checks

Marketing teams add thousands of leads daily. Sales onboarding pipelines generate batches of 10,000+ emails in minutes. Running a synchronous validation on that volume means waiting hours—even days—for results. Your system stalls, and downstream processes like campaign sends or CRM syncs sit idle.

Sync validation works for small batches. Once you hit 5,000+ records, latency becomes a bottleneck. The process isn’t just slow—it’s unsustainable. You can’t delay your email campaign while a validation job runs for 12 hours. That’s why you need async.

How async fits into modern data workflows

With async bulk validation, you submit a list, get an immediate response confirming the job is queued, then receive results later via a webhook or callback. This means your integration doesn’t wait. Your system can move on to the next task—data ingestion, segmentation, personalization—while validation runs in the background.

This approach is how high-throughput systems like data warehouses, ETL pipelines, and automation tools stay fast and reliable. It’s the standard for platforms handling real-time data at scale. As RFC 5321 (the SMTP core spec) clarifies, email validation is not a single query—each address must be checked at the MX level, via mail server responses, and in context of rate limits and greylisting. No single synchronous call can handle the full chain efficiently.

With tools like bulk email verification or real-time verification API, you don’t trade accuracy for speed. You get 98.9% accuracy across 100,000+ addresses—verified in minutes, not hours—via callbacks that update your CRM, database, or campaign system as the job completes.

Think of it like sending a shipment. You don’t wait for the delivery truck to return before starting the next batch. You dispatch, track status, and process receipts when they arrive. That’s what async validation with webhooks enables: continuous, reliable, and scalable email hygiene.

How webhooks ensure you don’t miss verification results

Without webhooks, you’d constantly poll your API just to check if a bulk verification job finished—wasting bandwidth and delaying your next step. With webhooks, Email List Validation sends an instant HTTPS POST to your server the moment a job completes or fails. That means your system can clean the data, flag invalid addresses, and resume delivery—all without waiting or manual checks.

Stop polling. Start reacting.

Imagine running a 100,000-email list clean. If you’re polling every 10 seconds just to see if it’s done, you could spend minutes or even hours checking status. That’s not just slow—it’s inefficient and unreliable. Every poll costs time and API tokens.

With webhooks, you set up a URL once, then forget. As soon as the job finishes (or encounters an error), Email List Validation sends a real-time signal. You’re not guessing—it’s a confirmed trigger. This is how systems at scale stay responsive.

According to the IETF’s RFC 7807, webhooks align with industry best practices for real-time event delivery, reducing latency and resource use compared to polling. This isn’t just convenience—it’s a fundamental shift in how systems communicate.

Instant actions, no delays

Once you receive the webhook, your system can act immediately. Valid emails go into your database. Invalid or risky addresses get flagged and excluded. If you use SendGrid, Mailchimp, or Klaviyo, the integration updates your audience in real time. No more manual exports or delayed cleanups.

Let’s say you’re preparing a campaign. Instead of waiting to check results, the webhook tells you the verification is complete—your clean list is ready and your sending pipeline stays active. That’s the difference between stalled workflows and uninterrupted outreach.

For the full workflow, see how it fits into your stack: bulk verification, real-time API, or integrated tools. You can start with 100 free verifications without expiration.

The role of callback URLs in automated email hygiene pipelines

When you send a large batch of emails for verification, Email List Validation uses a callback URL to deliver results once the process finishes—no polling, no waiting. This endpoint must be publicly accessible, use HTTPS, and accept structured JSON. You then decide what happens next: save it to your database, alert your team, or feed it into a BI system—all without pausing your workflow. It’s how automation becomes seamless.

How callback URLs work in practice

Let’s say you’re running a weekly list clean-up. You upload your list and specify a callback URL. When the batch finishes, Email List Validation sends the full result set as a JSON payload to that address. The system doesn’t wait for you to check in—it acts independently, like a trusted agent.

You control the logic after the fact. Maybe you log the result into a CRM. Maybe you trigger a Slack message if more than 10% of addresses are invalid. Maybe you pipe validated emails into a reporting dashboard. The URL is just the handoff point; what you do with the data is up to you.

What makes a callback URL effective

Your endpoint must be reachable from the public internet and use HTTPS. Unencrypted or private endpoints fail silently. It’s a technical requirement, not a suggestion. This is standard for modern APIs, as outlined in RFC 7540 (HTTP/2), and enforced by most security-conscious platforms.

Make sure the payload is structured consistently. Expect a JSON object with fields like batch_id, status, results, and timestamp. The results array will contain one object per email—each with a verdict (valid, invalid, catch-all, risky), reason, and metadata.

It’s helpful to design the endpoint for idempotency—handling retries without duplicating records. That’s a proven pattern in production systems, used by tools like AWS Lambda and Azure Functions to manage event-driven workflows.

For developers, this integration fits right into existing pipelines. If you’re using Mailchimp or HubSpot, you can wire the callback into a webhook trigger. Or, use the Email List Validation API for real-time checks during sign-up flows. Check out the integration guide to see how it works with your stack: integrations.

How to set up async validation with webhooks in three steps

Submit your list with async=true and a webhook URL, get a job ID instantly, then let Email List Validation process the list in the background. When done, it sends results to your webhook as JSON—no polling, no waits, just reliable, real-time data. This is how you scale verification without blocking your app.

  1. Send your list via the API with async=true and include your webhook URL. This tells Email List Validation to process the list in the background. Use our API to send the request. The system accepts your list and returns a job ID immediately—no delay while it verifies.
  2. Resume your app’s work right away. Since the job runs asynchronously, your application doesn’t need to wait. You can proceed with other tasks—like scheduling sends or updating a dashboard—without downtime. This is a standard pattern in scalable systems; think of it like sending an order to be processed in a queue instead of waiting for the oven to preheat.
  3. Receive results when done: a JSON payload delivered to your webhook. Once processing finishes, Email List Validation sends a full result set to your specified URL. The payload includes each email’s verdict—valid, invalid, catch-all, risky—and metadata like the verification status and reason. This eliminates polling, reduces latency, and keeps systems lean. You can integrate this into workflows used by teams managing hundreds of thousands of contacts daily.

Why async with webhooks matters for scale

When you’re verifying thousands of emails per batch, blocking your app while it waits isn’t just slow—it’s unsustainable. Asynchronous processing is standard in production systems, especially when dealing with external services. It reduces timeout risks and improves throughput. The RFC 5321 and RFC 5322 specifications define how email systems handle delivery, but they don’t cover API interactions—so you need tools that implement best practices, like avoiding hanging requests.

Webhooks are the reliable, scalable way to receive results. They’re supported by major platforms like AWS Lambda, Netlify, and Vercel, and are used across email, payment, and identity verification systems. You don’t need to constantly check status; the system notifies you when ready.

To begin, set up your webhook endpoint to accept POSTs. Then, use our API with async=true and send your list. Results arrive fast—often in seconds—when you're not polling.

Verdicts returned in webhook payloads: what they mean

You get clear, actionable verdicts in your webhook responses: valid (email is real and likely deliverable), invalid (format or domain error), catch-all (accepts all emails, but delivery can’t be confirmed), risky (disposable, role-based, or high bounce risk), or unknown (temporary failure at the recipient server). Each verdict tells you exactly what to do next—no guesswork.

What each verdict means in practice

  • valid: The email address exists and the domain accepts mail. This is the signal you want—likely to land in the inbox. Use it confidently in campaigns and workflows.
  • invalid: The format is broken—missing @, invalid TLD, or malformed structure. These are outright errors. Remove them before sending.
  • catch-all: The domain accepts messages for any address, but you can’t confirm if the specific email will be delivered. High risk of bounce. Mark these for manual review or exclude them.
  • risky: The email is likely disposable, role-based (e.g., admin@, sales@), or from a provider with poor deliverability. These have higher bounce rates and can hurt sender reputation. Filter them out or flag for verification.
  • unknown: The recipient server was unreachable or failed to respond in time. This isn’t a permanent failure—retry later. Don’t assume it’s invalid.

How these verdicts help you scale safely

Async bulk validation with webhooks lets you process thousands of emails without blocking your app. You get back structured verdicts, so you can automate rejection, filtering, or segmentation instantly.

For example, your system can send all valid addresses to your email service provider, quarantine risky ones for review, and skip invalid or unknown ones entirely.

Using SMTP-level checks and real-time delivery tests (like those in inbox placement testing), you avoid sending to addresses that bounce or get flagged—reducing strain on your sender reputation.

Industry best practice: always filter out catch-all and risky addresses during list hygiene. According to Spamhaus, poor list hygiene is a top trigger for blacklist placement.

Set up webhooks to handle these verdicts automatically—whether you're sending transactional emails, campaigns, or user onboarding flows. You’re not just validating; you’re guarding deliverability.

To start, run a bulk validation of your list today: clean your list at scale with instant feedback.

Webhook security: Protecting your callback endpoint

Always use HTTPS for your webhook endpoint to prevent data interception. Validate incoming requests with a secret token or HMAC signature to ensure they come from a trusted source. Log all requests and monitor for anomalies—unusual or repeated payloads can signal abuse or injection attempts. These steps are not optional; they’re essential when handling automated, high-volume validation results.

HTTPS is non-negotiable

Never expose your callback endpoint over HTTP. Any data sent in an unencrypted connection can be read or modified in transit. This includes sensitive verification results or user identifiers tied to email addresses. Use HTTPS with a valid certificate—tools like Let’s Encrypt make this easy and free.

Industry standards, like those outlined in RFC 6749, require secure transport for any endpoint accepting automated payloads. This isn’t just best practice—it’s a baseline for compliance with modern security expectations.

Verify the sender before acting

Even with HTTPS, you can't assume a request comes from your intended service. A malicious actor could spoof a valid-looking payload. To prevent this, configure your webhook with a secret token or HMAC signature. The service sending the webhook (like Email List Validation) signs each payload using a shared secret—a signature only your server can verify.

Let’s say you receive a callback: if the HMAC signature doesn’t match your locally calculated one, reject the request immediately. This stops replay attacks, forged payloads, and unintended triggers. It’s a lightweight but strong guardrail. You can implement this with any modern web framework—most provide built-in HMAC verification utilities.

Don’t skip validation just because the request came over HTTPS. Encryption protects the channel; authentication protects the data. Both are needed.

Log every incoming request—including timestamps, IP address, and payload size. This helps detect patterns of abuse, such as repeated POSTs from an unusual source. Sudden spikes in traffic to your endpoint, especially with invalid or malformed data, can indicate probing or automated scanning attempts.

Regularly audit these logs and set up alerts for abnormal behavior. You can find real-world guidance on anomaly detection in CIS Controls, which cover secure logging and monitoring as core security practices.

For context, our bulk verification service supports HTTPS and HMAC signing for your webhooks, ensuring you can integrate securely without guesswork.

Real-world use case: Onboarding new leads with zero delivery failures

When a SaaS platform receives 5,000 new sign-ups weekly, sending welcome emails to invalid or risky addresses causes soft bounces, hurts sender reputation, and wastes resources. By using async bulk validation with webhooks, they filter out bad emails before delivery. This setup prevents all soft bounces—zero failure—while keeping onboarding reliable at scale.

How async validation prevents delivery loss

Instead of checking emails synchronously, which pauses the onboarding pipeline, they queue validations asynchronously. This means the system processes each email in the background without blocking the workflow.

Once validation completes, a webhook notifies the application. You receive a real-time callback with the result: valid, invalid, catch-all, or risky. This allows your system to act immediately—filtering out bad addresses before the welcome campaign runs.

Because the process runs in bulk, a 5,000-email list can be validated in under 10 minutes. The system scales without overloading your servers. And since no emails are sent to invalid or risky addresses, you never trigger a soft bounce.

Precision meets performance

Unlike simple syntax checks, this approach tests actual deliverability: does the domain exist? Is the mailbox active? Could it be a disposable or role-based address? A well-configured system answers yes to these questions before anything ships.

For example, a catch-all domain may accept all incoming mail but doesn’t guarantee inbox placement. A risky address might be a role account (like admin@ or sales@), which often gets filtered or ignored. By flagging these early, you avoid sending to places where messages won’t land.

This is how real deliverability works. You’re not just validating syntax—you’re verifying that an email can receive mail, and that it’s likely to appear in the inbox.

Tools like bulk validation handle thousands of addresses at once, and the real-time API integrates with your pipeline to automate rejection of bad entries. A webhook ensures you’re informed when processing finishes, so your workflow continues without delay.

Industry standards like RFC 5321 define how SMTP servers should handle mail delivery. Following these rules—and proactively screening for non-compliant addresses—keeps your sender reputation intact.

When your onboarding system sends only to verified, valid addresses, every welcome email reaches its intended destination. No bounces. No delays. Just predictable, reliable delivery.

How Email List Validation handles failed or incomplete jobs

If a bulk validation job fails due to timeout, network issues, or a system error, Email List Validation automatically retries the job for up to six hours. Failure details—including error type, timestamp, and affected email addresses—are included in the webhook payload, so you can trigger fallback workflows like pausing your campaign or alerting your team. You can also review the full logs in the dashboard and re-submit the job directly.

Automatic retries with clear failure context

When a job doesn’t complete successfully, the system doesn’t give up. Instead, it attempts retries over a six-hour window, giving intermittent network issues or temporary DNS delays time to resolve. This is a standard practice in robust email infrastructure: RFC 5321 (SMTP) defines retry behavior for transient delivery errors, and systems like SendGrid or Amazon SES follow similar patterns.

Each retry is logged with timestamps and error codes. If the job fails after all attempts, the webhook payload includes full context—like whether the issue was a DNS timeout, invalid input format, or server-side failure—so you understand exactly what went wrong.

Recovery and review options

You can re-submit failed jobs with just a few clicks in the dashboard. Full logs show every step, including the specific email addresses that failed and why. This visibility is critical during campaign setup or compliance checks.

For example, if a job fails due to a malformed list or an oversized file, the logs will indicate it. You can then clean the data and re-submit. Unlike some tools that drop failed jobs entirely, we preserve them, so you don’t lose progress.

Let’s say you’re syncing a list from Mailchimp to your CRM. If the initial sync fails, your webhook can detect it, pause the pipeline, and notify you. You can fix the root cause and re-upload without starting over. It's not just about validation—it's about reliability in motion.

This process works with our bulk email list cleaning feature and integrates cleanly with tools like HubSpot, Klaviyo, or SendGrid through our integrations. Even with high-volume lists, you maintain control.

Why async validation isn't just fast—it improves deliverability

Async bulk validation with webhooks and callbacks ensures that only valid, deliverable addresses reach your inbox. By filtering out invalid, risky, and catch-all emails before sending, you reduce bounce rates and protect your sender reputation.

Email List Validation maintains a 98.9% accuracy rate across bulk verification jobs, meaning you can trust the results to identify truly problematic addresses. This precision helps maintain consistent sending patterns and avoids the penalties associated with high bounce or complaint rates.

Over time, clean lists lead to better inbox placement. ISPs favor senders with reliable engagement and low abuse signals. Removing risky addresses is not just about reducing errors—it’s about building long-term deliverability.

Keep reading

Ready to put this into practice? Email List Validation verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

How do webhooks work with bulk validation?

When a bulk verification job completes, Email List Validation sends a real-time HTTP POST to your predefined webhook URL with the full result set.

Can I use webhooks with the free tier?

Yes. You can use webhooks with your 100 free verifications—no extra cost, and credits never expire.

What happens if my webhook endpoint is unreachable?

Email List Validation retries up to 5 times over 12 hours. If still unreachable, the job status is marked 'failed' in your dashboard.

How do I test a webhook URL before live use?

Use the API's test endpoint or a web tool like ngrok to expose a local server and validate the payload format and response code.

Do I need to store webhook results locally?

Yes. Webhooks are not a backup—always log and store the results for audit, reporting, or reconciliation.

Can multiple webhooks be used per job?

No. Each job supports one webhook URL. Multiple endpoints must be handled by a single gateway or proxy server.

What’s the maximum size of a list I can verify async?

You can submit lists of any size—up to 100,000 emails per job, with automatic batching if needed.

Are webhook payloads encrypted?

Data is sent over HTTPS. The payload is not encrypted by us—but you can verify and encrypt it server-side.

How long does async validation take?

Most jobs finish in under 5 minutes. Larger jobs may take up to 30 minutes depending on load and validation depth.

Can I trigger different actions based on the result type?

Yes. Your webhook handler can parse the verdicts—e.g., route 'valid' to SendGrid, 'catch-all' to a review queue.

Does the API support real-time validation alongside async?

Yes. You can combine real-time API checks with async bulk jobs—ideal for hybrid workflows like onboarding and nightly cleaning.

How do I manage webhook secrets securely?

Store secrets in environment variables, not in code. Use HMAC signatures to validate payloads before processing.