Understanding Validation Speed

One of the most common questions we get is: “Why did my batch take so long?” The short answer is that email validation is not a simple database lookup – it involves real-time communication with mail servers around the world, and those servers don’t always cooperate. This article explains how Truelist processes your lists and why batches take the time they do.

How Batches Are Processed

When you upload a list, Truelist does not blast all emails to mail servers at once. Instead, emails are fed into the validation pipeline at a controlled rate. This rate limiting exists for good reason: mail servers will block or blacklist any source that connects too aggressively. To protect the accuracy of your results (and our infrastructure), we carefully manage how quickly emails are dispatched.

Here’s what happens:

1. Your list is uploaded and parsed. Duplicate and syntactically invalid emails are removed immediately.
2. Emails are dispatched in waves. Every few seconds, a new batch of emails is sent to the validation queue based on your plan’s processing speed.
3. Each email is validated individually. This involves connecting to the recipient’s mail server via SMTP, which is a real-time network operation.
4. Results are collected and written. Once all validations complete, results are finalized and your downloadable report is generated.

Processing Speed by Plan

Your plan determines how many emails per second are dispatched into the validation queue:

What This Means in Practice

For a 100-email list on a Growth plan (3 emails/second):

• Dispatch time: ~34 seconds to get all emails into the queue
• Per-email validation time: Each SMTP check typically takes 2-10 seconds, but some can take much longer
• Total time: Typically 1-3 minutes, but can be longer depending on the domains in your list

For a 10,000-email list on a Pro plan (10 emails/second):

• Dispatch time: ~17 minutes to get all emails into the queue
• Total time: 20-40 minutes is typical, depending on the mix of domains

Why Individual Emails Take Time

Each email validation is a multi-step process that involves real network connections:

1. DNS lookup – We look up the mail server (MX record) for the domain.
2. SMTP connection – We connect to the mail server and initiate a conversation.
3. Mailbox check – We ask the server whether the specific address exists.
4. Response handling – The server’s response is analyzed and classified.

This entire exchange happens over the internet in real-time. Most validations complete in a few seconds, but several factors can slow things down.

Greylisting

Some mail servers use a technique called greylisting where they temporarily reject the first connection attempt from an unknown sender. This is a spam prevention measure. When Truelist encounters a greylisted response, it must wait and retry later, which adds time to that particular validation.

Server Timeouts and Slow Responses

Not all mail servers respond quickly. Some are overloaded, misconfigured, or intentionally slow to respond. Truelist will wait for a response, but these slow servers add to overall batch time.

IP Rotation and Throttling

To maintain high deliverability and accuracy, Truelist rotates across multiple IP addresses when connecting to mail servers. If a mail server begins blocking or throttling one of our IPs, we:

• Switch to a different IP for that domain
• Apply cooldown periods to avoid further blocks
• Retry the validation from a fresh IP

This adaptive behavior produces more accurate results, but it takes additional time when mail servers are being defensive.

How Your Validation Strategy Affects Speed

The validation strategy you choose has a significant impact on how long your batch takes. Here’s how each strategy affects processing time:

Fast

The quickest strategy. Performs a single SMTP check with no retries. Greylisted or ambiguous results are returned as-is rather than retried. Choose this when speed matters more than reducing unknowns.

Accurate (Default)

Retries greylisted connections after a short delay. This catches emails that would otherwise be marked as unknown due to greylisting, but adds 30+ seconds per retried email.

Thorough

Extends the Accurate strategy with:

• Longer retry windows for greylisted servers (up to 5 minutes per retry)
• Additional retry attempts when IPs are temporarily throttled, waiting for cooldowns rather than giving up
• Capped processing speed at 10 emails/second (even on higher-tier plans)

This strategy can significantly increase batch time, but it produces the fewest unknown results.

Enhanced

The most comprehensive strategy. Includes everything in Thorough, plus:

• Third-party data cross-referencing for ambiguous results
• Additional verification techniques for uncertain emails

Enhanced batches will take the longest to complete, as uncertain emails go through multiple rounds of additional verification. This strategy uses enhanced credits.

Factors That Affect Total Batch Time

Here’s a summary of everything that influences how long your batch takes:

Domain Concentration

If your list has many emails at the same domain (e.g., 500 emails all at @example.com), validation for that domain will be throttled to avoid overwhelming or being blocked by that mail server. Lists with a diverse mix of domains typically process faster than lists concentrated at a single domain.

Caching

Truelist caches validation results. If an email was validated recently, the cached result is returned instantly without contacting the mail server again. Lists with many previously-validated emails will complete faster.

What to Expect

Here are rough time estimates for common scenarios. Actual times may vary based on the factors above.

Tips for Faster Results

• Use the Fast strategy if you don’t need retry behavior and can tolerate more unknown results.
• Avoid re-validating recently verified emails. Truelist caches results, so uploading the same list twice will be much faster the second time.
• Diversify the domains in your list. Lists heavily concentrated on a single domain (e.g., thousands of @example.com addresses) will be significantly slower due to per-domain throttling. If possible, batch your uploads so each list contains a mix of different domains rather than all addresses at one company.
• Split very large lists if you need partial results sooner. You can upload in segments and download results as each segment completes.
• Upgrade your plan if you regularly process large lists. Higher-tier plans dispatch emails significantly faster.

Still Have Questions?

If your batch seems unusually slow or stuck, contact us and we’ll look into it. Please include your batch ID so we can investigate the specific emails that are taking longer.