Peeking into Email Validation Techniques

ISAAC KIM

7 Oct 2019

Email

1 min read

Peeking into Email Validation Techniques

Key Takeaways

    • Email validation ensures an address exists, accepts mail, and belongs to the correct user.

    • Syntax validation was the earliest method, ensuring emails follow RFC-defined formatting rules.

    • ISP-level validation once existed via the VRFY SMTP command, but was disabled due to spam abuse.

    • SMTP Ping emerged as a workaround but is now treated as malicious behavior by ISPs.

    • SMTP Ping can generate false positives/negatives due to greylisting and delayed validation.

    • Modern best practices emphasize reducing typos, using double opt-in, and applying real-time validation tools.

    • Email validation tools help cut costs, improve deliverability, and reduce blocklist risk.

    • Data-driven validation avoids ISP connection abuse and relies on behavioral signals.

    • Data sources include hard bounces, deliveries, engagement, DNS checks, and domain quality.

    • SparkPost’s Recipient Validation leverages one of the industry’s largest email data footprints.

    • A single hard bounce alone is not enough to categorize an address as invalid—context matters.

    • Modern validation is shifting toward advanced machine learning and massive event-driven datasets.

Q&A Highlights

  • What is email validation and why is it important?

    Email validation verifies whether an address exists, can receive messages, and belongs to the intended user. It protects sender reputation, improves inbox placement, and increases the accuracy of campaign metrics.

  • What did early email validation look like?

    The earliest form was syntax validation, checking formatting rules based on RFCs—ensuring the local part, @ symbol, domain, and extension were correctly structured.

  • Why did syntax validation eventually fall short?

    Even syntactically correct addresses can be invalid or abandoned. Syntax rules alone can’t detect typos, inactive domains, or non-existent recipients.

  • What was SMTP VRFY and why was it disabled?

    SMTP VRFY allowed senders to ask a receiving server if an address was valid. It was disabled after widespread abuse by spammers who used it to harvest lists.

  • How did SMTP Ping work?

    SMTP Ping simulated sending an email, waited for recipient validation feedback, and disconnected before delivery. It was adopted as a workaround after VRFY disappeared.

  • Why is SMTP Ping considered harmful today?

    ISPs classify it as spammer behavior because it resembles directory harvesting. It can lead to hard blocks, reduced reputation, and even blocklisting.

  • What reliability issues does SMTP Ping have?

    It produces false positives (when validation is delayed until after the handshake) and false negatives (caused by greylisting or protective filters).

  • What are modern best practices for preventing typos?

    Use auto-complete suggestions for common domains and detect misspellings like @gmsil.com or @yaho.com before users submit forms.

  • Why use double opt-in?

    Double opt-in ensures the address belongs to the user, reduces complaints, aligns with regulations, and validates that the mailbox accepts mail.

  • What are the benefits of using advanced validation tools?

    They save time and money, reduce bounce rates, prevent blocklist issues, improve deliverability, and provide real-time results across user touchpoints.

  • What is data-driven validation?

    It uses a large historical dataset—hard bounces, deliveries, opens, clicks, domain checks, disposable-email detection—to determine whether an address is safe to send to without relying on ISP responses.

  • How does SparkPost’s Recipient Validation differ?

    It’s built on SparkPost’s massive sending footprint (over 37% of global B2C/B2B email), letting its models interpret bounces in context and continuously refine validity assessments at scale.

From regular expression conformity to today’s sophisticated tools, email validation has gone through a few different eras. Email itself has undergone remarkable transformations since its inception. For some entertaining insights into email's journey, explore our 13 fun email facts you might not know. Before looking at the history of validation techniques and best practices, it’s essential to understand the basics.

From regular expression conformity to today’s sophisticated tools, email validation has gone through a few different eras. Email itself has undergone remarkable transformations since its inception. For some entertaining insights into email's journey, explore our 13 fun email facts you might not know. Before looking at the history of validation techniques and best practices, it’s essential to understand the basics.

From regular expression conformity to today’s sophisticated tools, email validation has gone through a few different eras. Email itself has undergone remarkable transformations since its inception. For some entertaining insights into email's journey, explore our 13 fun email facts you might not know. Before looking at the history of validation techniques and best practices, it’s essential to understand the basics.

Why Use Email Validation Tools?

Many reasons exist for using email validation tools (EVs), including:

  • It saves you time: EVs rely on automated processes for validation that reduce the time associated with manual look-ups.

  • It saves you money: A clean email list with valid addresses eliminates bounces and increases ROI by reducing unnecessary spending.

  • It keeps you off the blocklist: Using EVs lowers spam complaints that can lead to message and sender blocking.

  • It improves deliverability: Readers need to receive your emails before interacting with them, so deliverability is a priority for promotional and transactional communications.

  • It offers real-time results: Being able to validate immediately across different points of the buyer’s process supplies you with higher-quality contact information.

Many reasons exist for using email validation tools (EVs), including:

  • It saves you time: EVs rely on automated processes for validation that reduce the time associated with manual look-ups.

  • It saves you money: A clean email list with valid addresses eliminates bounces and increases ROI by reducing unnecessary spending.

  • It keeps you off the blocklist: Using EVs lowers spam complaints that can lead to message and sender blocking.

  • It improves deliverability: Readers need to receive your emails before interacting with them, so deliverability is a priority for promotional and transactional communications.

  • It offers real-time results: Being able to validate immediately across different points of the buyer’s process supplies you with higher-quality contact information.

Many reasons exist for using email validation tools (EVs), including:

  • It saves you time: EVs rely on automated processes for validation that reduce the time associated with manual look-ups.

  • It saves you money: A clean email list with valid addresses eliminates bounces and increases ROI by reducing unnecessary spending.

  • It keeps you off the blocklist: Using EVs lowers spam complaints that can lead to message and sender blocking.

  • It improves deliverability: Readers need to receive your emails before interacting with them, so deliverability is a priority for promotional and transactional communications.

  • It offers real-time results: Being able to validate immediately across different points of the buyer’s process supplies you with higher-quality contact information.

What Is Email Validation and Why Is It Important?

Email validation is a method for verifying that a recipient’s address exists and accepts messages. Some techniques take validation an additional step by confirming the address belongs to the intended person, and the recipient wants to receive such communications. During the process, you’ll eliminate risky and invalid addresses from your list and protect your name as a reputable sender with Internet Service Providers (ISPs).

What Is Email Validation and Why Is It Important?

Taking care to validate user emails before sending marketing and transactional messages also increases the chances that your communications route directly to a reader’s inbox. In addition, campaign metrics are more accurate and provide deeper insights for future data-driven decisions.

Email validation is a method for verifying that a recipient’s address exists and accepts messages. Some techniques take validation an additional step by confirming the address belongs to the intended person, and the recipient wants to receive such communications. During the process, you’ll eliminate risky and invalid addresses from your list and protect your name as a reputable sender with Internet Service Providers (ISPs).

What Is Email Validation and Why Is It Important?

Taking care to validate user emails before sending marketing and transactional messages also increases the chances that your communications route directly to a reader’s inbox. In addition, campaign metrics are more accurate and provide deeper insights for future data-driven decisions.

Email validation is a method for verifying that a recipient’s address exists and accepts messages. Some techniques take validation an additional step by confirming the address belongs to the intended person, and the recipient wants to receive such communications. During the process, you’ll eliminate risky and invalid addresses from your list and protect your name as a reputable sender with Internet Service Providers (ISPs).

What Is Email Validation and Why Is It Important?

Taking care to validate user emails before sending marketing and transactional messages also increases the chances that your communications route directly to a reader’s inbox. In addition, campaign metrics are more accurate and provide deeper insights for future data-driven decisions.

A long time ago in an era far, far away… it started with Syntax Validation

Checking an email address for syntax accuracy has been the simplest version of email validation. The core elements of a valid email address are the local part, the @ symbol, the domain, and finally, the extension (.com, .org, etc.). To help standardize all the various syntaxes, specifications called Requests For Comments (RFCs) were published to determine what characters would be acceptable for local and domain parts. These RFCs eventually became quite extensive and created the need for open-source libraries to help validate email syntax in many languages.

Another vital component of syntax validation is apparent misspellings, such as heyitsme@gmsil.com. Proofing your existing list for these types of errors remains a best practice.

Checking an email address for syntax accuracy has been the simplest version of email validation. The core elements of a valid email address are the local part, the @ symbol, the domain, and finally, the extension (.com, .org, etc.). To help standardize all the various syntaxes, specifications called Requests For Comments (RFCs) were published to determine what characters would be acceptable for local and domain parts. These RFCs eventually became quite extensive and created the need for open-source libraries to help validate email syntax in many languages.

Another vital component of syntax validation is apparent misspellings, such as heyitsme@gmsil.com. Proofing your existing list for these types of errors remains a best practice.

Checking an email address for syntax accuracy has been the simplest version of email validation. The core elements of a valid email address are the local part, the @ symbol, the domain, and finally, the extension (.com, .org, etc.). To help standardize all the various syntaxes, specifications called Requests For Comments (RFCs) were published to determine what characters would be acceptable for local and domain parts. These RFCs eventually became quite extensive and created the need for open-source libraries to help validate email syntax in many languages.

Another vital component of syntax validation is apparent misspellings, such as heyitsme@gmsil.com. Proofing your existing list for these types of errors remains a best practice.

Validation SMTP Command and The Attack of the Spammers

Recognizing the need for help on validating email addresses, Internet Service Providers (ISPs) started to build in email address validation functionality. Thus, “VRFY” (also known as Verify) was built as an SMTP command which enabled senders to ask a receiving mail server if an email address was valid. With the hope to use VRFY to bring peace and order to the galactic Internet, it soon fell into the wrong hands of the dark side; spammers. After wide-scale abuse of this functionality, ISP administrators disabled VRFY, leaving email address validation in disarray.

Meme

Recognizing the need for help on validating email addresses, Internet Service Providers (ISPs) started to build in email address validation functionality. Thus, “VRFY” (also known as Verify) was built as an SMTP command which enabled senders to ask a receiving mail server if an email address was valid. With the hope to use VRFY to bring peace and order to the galactic Internet, it soon fell into the wrong hands of the dark side; spammers. After wide-scale abuse of this functionality, ISP administrators disabled VRFY, leaving email address validation in disarray.

Meme

Recognizing the need for help on validating email addresses, Internet Service Providers (ISPs) started to build in email address validation functionality. Thus, “VRFY” (also known as Verify) was built as an SMTP command which enabled senders to ask a receiving mail server if an email address was valid. With the hope to use VRFY to bring peace and order to the galactic Internet, it soon fell into the wrong hands of the dark side; spammers. After wide-scale abuse of this functionality, ISP administrators disabled VRFY, leaving email address validation in disarray.

Meme

SMTP Ping (The Spammer Menace)

SMTP Ping (The Spammer Menace)

After the fall of VRFY, senders creatively devised SMTP Ping, a different method to verify whether or not an email address was valid. SMTP Ping would be used to check against a remote mail server to see if an email address was alive. A connection to the Internet Service Provider (ISP) remote mail server, such as Gmail, would be made as if actually sending an email, but abruptly cut short without actually sending the email.

Typically, the conversation held in the connection between the sending mail server and the receiving ISP mail server would look like this:

Diagram showing the SMTP email delivery conversation between sending and receiving mail servers.


In some scenarios, the ISP could provide feedback like this instead:

Illustration of SMTP email rejection scenario where the receiving mail server responds negatively to a delivery attempt for a specific email address.


With SMTP Ping, senders could cut the conversation short after seeing the response back from the ISP, after requesting to send mail to the specified email address. This became a way to ping against an ISP to see if the receiving mail server found the email addresses to be valid or invalid, with some degree of confidence.

Meme
SMTP Ping (The Spammer Menace)

After the fall of VRFY, senders creatively devised SMTP Ping, a different method to verify whether or not an email address was valid. SMTP Ping would be used to check against a remote mail server to see if an email address was alive. A connection to the Internet Service Provider (ISP) remote mail server, such as Gmail, would be made as if actually sending an email, but abruptly cut short without actually sending the email.

Typically, the conversation held in the connection between the sending mail server and the receiving ISP mail server would look like this:

Diagram showing the SMTP email delivery conversation between sending and receiving mail servers.


In some scenarios, the ISP could provide feedback like this instead:

Illustration of SMTP email rejection scenario where the receiving mail server responds negatively to a delivery attempt for a specific email address.


With SMTP Ping, senders could cut the conversation short after seeing the response back from the ISP, after requesting to send mail to the specified email address. This became a way to ping against an ISP to see if the receiving mail server found the email addresses to be valid or invalid, with some degree of confidence.

Meme
SMTP Ping (The Spammer Menace)

After the fall of VRFY, senders creatively devised SMTP Ping, a different method to verify whether or not an email address was valid. SMTP Ping would be used to check against a remote mail server to see if an email address was alive. A connection to the Internet Service Provider (ISP) remote mail server, such as Gmail, would be made as if actually sending an email, but abruptly cut short without actually sending the email.

Typically, the conversation held in the connection between the sending mail server and the receiving ISP mail server would look like this:

Diagram showing the SMTP email delivery conversation between sending and receiving mail servers.


In some scenarios, the ISP could provide feedback like this instead:

Illustration of SMTP email rejection scenario where the receiving mail server responds negatively to a delivery attempt for a specific email address.


With SMTP Ping, senders could cut the conversation short after seeing the response back from the ISP, after requesting to send mail to the specified email address. This became a way to ping against an ISP to see if the receiving mail server found the email addresses to be valid or invalid, with some degree of confidence.

Meme

The Dark Side of SMTP Ping

ISPs consider SMTP Ping — also known as the broken handshake — as spammer behavior. ISPs can easily tell that you’re just checking validity of email addresses by looking at the conversation patterns: Calling in and hanging up repetitiously, with no (or very little) messages actually being sent, ends up in their mail server logs. After the history with SMTP VRFY, this type of behavior is now known to be spammy. ISPs are cracking down on this behavior and cracking down hard. Microsoft for example, considers this type of practice to be malicious and Hotmail finds SMTP Ping as evidence of a directory harvest attack. SMTP Ping attempts in progress will typically drop a hard block on all connections from the sending IP address. ISPs dislike SMTP Ping, and so do blacklist operators. Keep it up, and you’ll almost surely end up getting blacklisted.

In addition, many ISPs delay recipient validation until after the server conversation ends, resulting in an acceptance without confirmation of email address validity — a false positive. Some ISPs also protect recipients with greylisting. Under grey list protocols, the receiving server rejects emails from unknown senders. As a result, SMTP Pings will return an error message even on valid email addresses — a false negative.

Long story short, it’s a really bad practice, and it’s unreliable.

ISPs consider SMTP Ping — also known as the broken handshake — as spammer behavior. ISPs can easily tell that you’re just checking validity of email addresses by looking at the conversation patterns: Calling in and hanging up repetitiously, with no (or very little) messages actually being sent, ends up in their mail server logs. After the history with SMTP VRFY, this type of behavior is now known to be spammy. ISPs are cracking down on this behavior and cracking down hard. Microsoft for example, considers this type of practice to be malicious and Hotmail finds SMTP Ping as evidence of a directory harvest attack. SMTP Ping attempts in progress will typically drop a hard block on all connections from the sending IP address. ISPs dislike SMTP Ping, and so do blacklist operators. Keep it up, and you’ll almost surely end up getting blacklisted.

In addition, many ISPs delay recipient validation until after the server conversation ends, resulting in an acceptance without confirmation of email address validity — a false positive. Some ISPs also protect recipients with greylisting. Under grey list protocols, the receiving server rejects emails from unknown senders. As a result, SMTP Pings will return an error message even on valid email addresses — a false negative.

Long story short, it’s a really bad practice, and it’s unreliable.

ISPs consider SMTP Ping — also known as the broken handshake — as spammer behavior. ISPs can easily tell that you’re just checking validity of email addresses by looking at the conversation patterns: Calling in and hanging up repetitiously, with no (or very little) messages actually being sent, ends up in their mail server logs. After the history with SMTP VRFY, this type of behavior is now known to be spammy. ISPs are cracking down on this behavior and cracking down hard. Microsoft for example, considers this type of practice to be malicious and Hotmail finds SMTP Ping as evidence of a directory harvest attack. SMTP Ping attempts in progress will typically drop a hard block on all connections from the sending IP address. ISPs dislike SMTP Ping, and so do blacklist operators. Keep it up, and you’ll almost surely end up getting blacklisted.

In addition, many ISPs delay recipient validation until after the server conversation ends, resulting in an acceptance without confirmation of email address validity — a false positive. Some ISPs also protect recipients with greylisting. Under grey list protocols, the receiving server rejects emails from unknown senders. As a result, SMTP Pings will return an error message even on valid email addresses — a false negative.

Long story short, it’s a really bad practice, and it’s unreliable.

May the Verifications Be With You — Best Practices for Email Validation

There are various steps you can take to ensure your approach to validating emails is robust, including:

  • Get proactive with typos: Consider adding an auto-complete tool to your email entry field with popular @example.com extensions, such as @gmail.com. Allowing users to select from the preformatted entries reduces the potential for a typo.

  • Go for the double opt-in: Sending an email confirming a user request that requires them to verify their email satisfies regulations for certain markets and provides validation.

  • Take advantage of technology: Various email verification tools offer an easy, affordable way to maximize communication potential and maintain friendly relationships with ISPs.

  • Establish a routine: While it may be tempting to validate emails only when executing a bulk-send campaign, regularly scheduling a validation may be the better option. Consider how often and from where emails join your list — the more options for capturing contact information, the more potential for invalid addresses.


Informational graphic explaining the benefits of email validation tools, showing they save time and money, prevent blocklisting, improve deliverability, and provide real-time results.

There are various steps you can take to ensure your approach to validating emails is robust, including:

  • Get proactive with typos: Consider adding an auto-complete tool to your email entry field with popular @example.com extensions, such as @gmail.com. Allowing users to select from the preformatted entries reduces the potential for a typo.

  • Go for the double opt-in: Sending an email confirming a user request that requires them to verify their email satisfies regulations for certain markets and provides validation.

  • Take advantage of technology: Various email verification tools offer an easy, affordable way to maximize communication potential and maintain friendly relationships with ISPs.

  • Establish a routine: While it may be tempting to validate emails only when executing a bulk-send campaign, regularly scheduling a validation may be the better option. Consider how often and from where emails join your list — the more options for capturing contact information, the more potential for invalid addresses.


Informational graphic explaining the benefits of email validation tools, showing they save time and money, prevent blocklisting, improve deliverability, and provide real-time results.

There are various steps you can take to ensure your approach to validating emails is robust, including:

  • Get proactive with typos: Consider adding an auto-complete tool to your email entry field with popular @example.com extensions, such as @gmail.com. Allowing users to select from the preformatted entries reduces the potential for a typo.

  • Go for the double opt-in: Sending an email confirming a user request that requires them to verify their email satisfies regulations for certain markets and provides validation.

  • Take advantage of technology: Various email verification tools offer an easy, affordable way to maximize communication potential and maintain friendly relationships with ISPs.

  • Establish a routine: While it may be tempting to validate emails only when executing a bulk-send campaign, regularly scheduling a validation may be the better option. Consider how often and from where emails join your list — the more options for capturing contact information, the more potential for invalid addresses.


Informational graphic explaining the benefits of email validation tools, showing they save time and money, prevent blocklisting, improve deliverability, and provide real-time results.

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© 2025 Bird

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A person is standing at a desk while typing on a laptop.

The complete AI-native platform that scales with your business.

Contact sales

Start for free

© 2025 Bird

Contact Support