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This guide provides further details of the Email Validation response fields in order to aid interpretation.

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IsGood Values - Additional Notes

Here is a more detailed breakdown:
Returned ValueDescription
Email is GoodIndicates with high confidence that the email address is deliverable and good. The email address was verified with the host mail server and no malicious warnings were found.
Email is Probably GoodIndicates that the email is deliverable but one or more minor warnings were found. For example the email may be a potential alias or a role, which are sometimes used as disposable addresses.
UnknownIndicates that not enough information was available to determine deliverability and integrity. Unknowns most commonly occur for slow mail servers that do not respond to the web service in time. They also occur for catch-all mail servers and greylists.
Email is Probably BadIndicates that one or more warnings were found, such as a potential vulgarity or a string of garbage-like characters.
Email is BadIndicates with high confidence that the email address is bad and/or undeliverable. Occurs for email addresses that fail critical checks such as syntax validation and DNS verification. Most commonly occurs for email addresses where the actual host mail server verified that the email does not exist. Also occurs for deliverable email addresses that are known spam traps or bots.

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Other Additional Flag Detail

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IsSMTPServerGood

Returns true, false or unknown to indicate if the email’s host mail server was responsive at the time of the check. This is a one of the service’s critical checks. If this value comes back false then it will be reflected in the IsDeliverable value and in the IsGood. Refer to this value if the email is unknown. If the value for this field is also unknown then the service most likely did not have enough time to finish verifying the email address with its host mail server. In these cases the service will continue to try and verify the email in a background process even though the request has finished. Chances are high that if you wait one or more hours and check the email again that the service will have been able to finish verifying the email addresses with the host mail server.

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IsCatchAllDomain

Returns true, false or unknown to indicate if the email’s host mail server is a catch-all. A catch-all mail server will say that an email address is deliverable even if it is not. This is because catch-all mail servers do not reject email addresses during the initial SMTP session. This means that a catch-all mail server cannot be trusted to verify the deliverability of an email address because it may or may not reject the email address until after an email message is sent. If an email address is unknown and this value is false then chances are good that if the email is checked again at a later time then the service will have verified its deliverability. If catchall is true and there are no warnings, then we know that the domain and mail server are good but the mail box can’t be verified by the mail server. In general this scenario leads to a 55% chance that the email is deliverable and won’t result in a hard bounce.

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IsSMTPMailBoxGood

Returns true, false or unknown to indicate if the service was able to verify the email address with its host mail server. This value can be treated similarly to the IsDeliverable value. A true value indicates that the email address is deliverable. If the value comes back false then the mail server verified that the email is undeliverable. A false will be accompanied by the warning flag. Some common reasons why this value will return unknown;
  • the mail server is a catch-all
  • the service ran out of time when communicating with the host mail server
  • the host mail server used a defensive tactic such as a greylist.

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Descriptive Flag Detail

The result fields given above are useful when it comes to sorting, grouping and filtering all of your validated email addresses. Next, we will look at some of the descriptive flags that the service may return. These flags can be used programmatically or at a glance to determine the status of an email address. There are many warning flags that the service may return but we will look at some of the more common and critical ones.

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DisposableEmail, SpamTrap, KnownSpammer and Bot

An email address may be deliverable but if one or more of these warning flags is returned then it is highly recommended to reject it.

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Alias, Bogus and Vulgar

If one of these warning flags is returned then you may want to either reject the email or set it aside for later review, depending on how strict you want to be.

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InvalidSyntax, InvalidDomainSpecificSyntax and InvalidDNS

These are warnings for critical checks that failed.

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Note Codes and Descriptions

The note flags will return descriptive information about the email, not all of which will affect the IsGood response, but we will focus on the ones that will explain why some email addresses came back as unknown.

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GreyListed

The service is good at detecting greylist behavior from mail servers and has procedures in place to avoid them, but not all greylists are avoidable. If the service encounters a greylist then it is temporarily unable to verify the email address with its host mail server. If you encounter a greylist then chances are good that if you try to validate the email again a couple of hours later that you will get a better response. Domains flagged as “KnownGreylisters” are identified by an algorithm that tracks when the service encounters a grey listing technique when attempting to verify an email addresses.

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MailServerTemporarilyUnavailable

This flag indicates that the service was able to connect to the email’s host mail server, but that the server was temporarily busy or unavailable and it was unable to verify the email for us. If you encounter this flag, try to validate the email again a few of hours later to see if the server becomes more responsive.

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SpamTrap and “KnownSpammer”

“SpamTrap” and “KnownSpammer” are identified by checking the email and IP against various community driven services that are comprised of thousands of webmasters and administrators that are dedicated to fighting spammers.

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ServerConnectTimeout

This flag indicates that the service was unable to establish a connection with a host mail server. A possible reason for the connection failure could be that the mail server is completely offline or it is responding too slow and unable to respond in time. Some mail servers are configured to commonly respond slowly, taking as long as 60 seconds to respond to a connection. This behavior is rare but it is not entirely uncommon. If an email returns this flag then try and enter a longer timeout time to allow the service the time it needs to verify the email.

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MailBoxTimeout

This flag indicates that the service was unable to finish verifying the email address with the host mail server in the time allowed. The mail server could be responding very slowly or the timeout time given to the service was too short. If an email returns this flag then try and enter a longer timeout time to allow the service the time it needs to verify the email.

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BlackListedDomain

The “BlacklistedDomain” check works by using multiple services to perform a lookup on the domain and IP. This is very similar to how many managed firewalls work that have subscriptions to blacklists to help keep them up to date.

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KnownComplainer

The “KnownComplainer” is identified by checking the email against a list comprised of multiple Email Feedback Loops (FBL).

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Bot

Indicates that the email address has been reported as being a known bot. It’s not unusual for valid email addresses to be created to purely be used as bots that then go on a spam-spree that then end up going bad because they have been disabled for their malicious activity. Usually the mailboxes get listed or simply fill up from a large amount of backscatter or rejection bouncebacks until they are no longer usable. At which point the malicious user/application moves on to the next email to use as a spam bot.