How Apollo Verifies Emails

Article author
Sarah Malone


If you've used an email verification tool like Neverbounce, Zerobounce, Briteverify, or another similar tool, you may wonder why they sometimes flag Apollo-verified emails as unverifiable. In a nutshell, here's why:

Most third-party tools can identify if a domain is active. Some can confirm if a domain is catch-all. But very few can detect at the mailbox level whether a specific email address from a catch-all domain exists or not.

This is because the majority of popular tools use a method called SMTP tickling.

Hold the Phone, What's that?

Simple Mail Transfer Protocol (SMTP) is a Transmission Control Protocol/Internet Protocol (TCP/IP) for sending messages. Most email verification tools leverage SMTP tickling to ask an email service if an email exists, thereby verifying its validity before you send any messages to it.

SMTP tickling cannot differentiate between valid and invalid emails from a catch-all domain. Therefore, this method cannot confirm if the email address does exist and will link you to the right person or if your messages will be auto-dumped into a default inbox that no one will see.

Catch-all domains can still lead you to qualified contacts. However, when you use most third-party verification tools it's very difficult to guarantee they will. No one wants to gamble with your domain reputation. Hence the catch-22 dilemma: include these emails in your email outreach and you put your domain at risk. Leave them out and you risk losing a chunk of potentially qualified leads.

Unless you use Apollo. We do things differently.

What Apollo Does Differently

At Apollo, we have a built-in email verification process with a 91% accuracy rate that does not rely solely on SMTP tickling to validate emails. Our 7-step email verification process can differentiate between valid and invalid emails from catch-all domains. This means that when you send an email to an Apollo-verified contact with a catch-all domain, your email will land in the right inbox.

Feeling skeptical?

We feel you. You don't want to put your domain at risk—and quite rightly too. Remember though—your success is our success. We value every one of our Apolloers and do not gamble with your reputation. Check out the FAQs below or reach out to our friendly support team to discuss any concerns you may have.

So what makes us different from other verification tools? As a B2B database provider, lead data is the bread and butter of Apollo's business. Providing accurate data and verifying emails is integral to the success of all Apollo users. To maintain the highest level of data consistency and accuracy, Apollo has a 7-step email verification process in place. This process takes advantage of Apollo's large datasets.

Apollo Datasets

Apollo collects and verifies data against multiple data sources to identify the best source of truth. Data sources include, but are not limited to:

  • The Apollo data contributor network
  • Public data crawling
  • Carefully vetted third-party data providers

By running checks against our large datasets, Apollo effectively verifies emails in real time, handles catch-all domains, predicts email bounces, and automatically cleans emails. Third-party email verification tools cannot leverage such insights to gain the same level of accuracy and granularity when verifying emails—especially when they are from catch-all domains.

The 7-Step Apollo Process

Apollo goes one step further than most verification tools to confirm that an account may or may not exist,

Many users email prospects daily through Apollo to find and contact qualified leads and track the deliverability and success of their sequences. This means that Apollo does not only use advanced technologies to run syntax, domain, and email checks to determine the validity of each email. Apollo also leverages its insightful data to check and confirm if a real person is actively using an email. Within the 7-step email verification process, Apollo can:

  • Check and validate emails against Apollo-connected inboxes, CRMs, and CSV uploads
  • Track email delivery statistics
  • Monitor email bounce-rates

Therefore, when you request and spend 1 credit on a verified email in Apollo, you can be sure of 2 important points:

  • Apollo can provide greater email coverage for contacts vs. third-party email verification services because we do not eliminate a valid email just because it belongs to a catch-all domain.
  • Apollo has gathered ample evidence that the email does exist and will reach an actual person's inbox
Curious about Credits?

For more information about how credits work in Apollo, please refer to the "What Are Credits?" article.

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Follow Email Best Practices

Now that you have read about the Apollo verification process and know why you may encounter conflicting results between Apollo-verified data and third-party verification tools, it's time to address the elephant in the room: Sending messages to verified emails is essential, but it's only one part of the magic potion. Even the best verification tool is not an all-encompassing solution. Several factors impact your bounce rate, including low open rates and recipients flagging your emails as SPAM.

If you want to make sure you're doing everything you can to keep your domain healthy and improve your deliverability rates when sending emails through Apollo, feast your eyes on these resources:

More questions Running Through Your Magnificent Mind?

We know. There's a lot to take in. If you still have questions, hop on down to the FAQ section below and skip straight to the answers you're looking for.

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What Does Catch-All Mean?

A catch-all email server is set up to accept every email sent to that domain—even if the email address does not exist.

It's Polyonymous!

You may have heard of the terms "accept-all emails" or "wildcard email aliases" too. Fear not young padawan—catch-all emails go by many names but the same definition applies to them all.

Here's an example of how unverified catch-all emails can be problematic for your campaigns:

Domain Name Existing Email Addresses from Example Domain Catch-All Inbox Invalid Email Address,,,

You have an email for Terry at Example. His email is However, Terry stopped working at Example 1 month ago and the company is yet to replace him. Example has 4 existing employee email addresses, plus 1 catch-all email address. This means that even though Terry left the company and his email address is no longer valid, the message you send to will land in the catch-all inbox.

With the rise in spammers taking advantage of catch-all email domains, only some companies still dedicate resources to keep on top of reviewing the emails in their catch-all inboxes. If you send a message to an invalid email from a catch-all domain and your message lands in an inbox that no one checks regularly, it's unlikely anyone will see it. It's even less likely the specific person you are trying to reach will open or read it. This can lead to lower open rates and over time, can affect your domain reputation.

However, many companies have a catch-all email server because they allow important emails to reach a company inbox, despite sender errors. So let's say Terry never left the company and his email does still exist. If you don't email him, you'll miss out on a potential deal just because his email address belongs to a catch-all domain.

Luckily, at Apollo, we can distinguish between valid and invalid emails from catch-all domains. Refer to the What Apollo Does Differently section above for more info.

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Why Do Third-Party Tools Flag Apollo-Verified Emails as Unverifiable?

In short, as lead data is the bread and butter of Apollo's business, we can collect and verify data against multiple data sources that third-party verification tools cannot. We do not solely rely on SMTP tickling to verify if a mailbox exists, which means we can distinguish between valid and invalid emails from catch-all domains when other verification tools cannot. As a result, many third-party tools flag valid, Apollo-verified emails as unverifiable because they belong to a catch-all domain.

For a more detailed explanation, refer to the first half of this article.

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Do I Need a Third-Party Email Verification Tool to Verify My Apollo Emails?

You do not. Apollo has an advanced, built-in, 7-step verification process with a 91% accuracy rate.

Providing accurate data and verifying emails is integral to the success of all Apollo users, and we are committed to your success.

For more information on our verification process, refer to the 7-Step Apollo Process section above.

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How Do I Keep My Domain Healthy?

Start by watching and implementing the tips from the webinar below into your outreach strategy. In this video, you'll learn fundamental steps to increase email deliverability, keep your domain safe, and avoid the dreaded SPAM folder.


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What Does a 91% Accuracy Rate Actually Mean?

Justice Bounces Back!

Fear not, for fair play reigns free at Apollo! If you send an email to an Apollo-verified contact within 30 days of requesting the details and the email bounces, Apollo refunds your email credit. For more information, please refer to the "Check Your Email Credit Refunds" section of the "What Are Credits?" article.

Apollo-verified emails have an accuracy rate of 91%. Although it may be impossible to eradicate invalid emails entirely from your prospecting workflow, many factors contribute to Apollo's 9% bounce rate. Due to our thorough, tried-and-tested verification process, invalid emails make up a very small portion of that percentage. Other factors include:

  • If the recipient has configured their email settings to block incoming emails
  • If the recipient's mailbox is full
  • If the recipient's server is temporarily unresponsive
  • If you have a negative email domain reputation.

To keep track of your sequences in Apollo, make sure you leverage email analytics to monitor email performance and run A/B tests to keep improving your open, click, and response rates. You can also take advantage of the sequence diagnostics tool to pinpoint any issues with your domain.

Looking For Email Tips?

For inspiration on how to increase your conversion rates, refer to the "How Do I Write Effective Sales Emails?" article for a brief introduction. If you want to increase your open rates but don't know where to start, hop into this blog post for a comprehensive list of effective subject line examples to get your creative juices flowing.

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