Frequently asked questions

Can I trust you with my data?

Great question. See Compliance for more information.

How can I get a key?

Click here to signup.

How do I call the API?

For a JSON response, make a simple GET request to the endpoint. For example, to query email address with license key ABCD1234 call:


Several response formats (other than JSON) are available. For a detailed explanation of the responses available, see Schema.

What comes back from the API?

Various text or binary response formats.


For a detailed explanation of the responses available, see Schema.

How reliable is the API?

> 99.9% average availability with a defined SLA. See Service reliability

Does the system get slower when it’s busy?

No. All infrastructure is hosted in cloud based platforms with automatic scaling enabled. Automatic scaling kicks in at busy times to provide more hardware resources to meet demand.

Do you cache results?

To deliver the speed and reliability demanded by our customers, verification results are cached as follows:

No personally identifiable information is stored in our cache infrastructure.

Can I get my usage in real-time?

Yes. Please see Usage report for more information.

Can it do Hotmail?


Can it find spam traps?


A Spam Trap is a moving target. In theory (and indeed in practice) anyone can setup a Block List and start putting spam traps into the wild.

Email Hippo has Spam Trap detection capabilities that covers several of the well known block lists. Whilst it is not possible to deliver 100% coverage of all spam traps from all block lists, Email Hippo provides the best Spam Trap detection capabilities available.

How does it work?

At a basic conceptual level, the process of verifying email addresses is very simple. Google for “Send email using telnet” for a quick and general overview of how it’s done. To verify an email address without sending an email, simply go as far as the “RCPT TO” stage and parse the response code. That’s the easy bit and can be accomplished in just a couple of dozen lines of a PHP script!

The hard bit is dealing with mail services that are intrinsically configured to work against the process of email verification or any similar SMTP based activity. The reason that any email / SMTP process is difficult from a client perspective is that mail services need to protect themselves from an ever increasing landscape of abuse including spam and DDoS attacks.

Email Hippo’s strength in dealing with the “hard bit” of email verification comes from years of experience in doing email verification together with our complete ownership of our SMTP verification software stack together with an extensive cloud based infrastructure. That’s why Email Hippo can do the “hard bits” best and offer outstanding coverage on the more difficult domains such as Yahoo and Hotmail.

Can I get blacklisted using this API?

No. It’s Email Hippo infrastructure that does the work.

Will anyone know that I am verifying their email address?

No. It’s Email Hippo infrastructure that does the work.

Your service says an address is OK and I know it’s Bad (or vice versa)?

Email Hippo queries mail servers in real time. Mail servers respond with one of two possible answers for a given email address:

  • Yes, the email address exists - SMTP code 2xx

  • No, the email address does not exist - SMTP code 5xx

Email Hippo uses the above response codes to determine if an email address is valid or not and reports this back to you.

This method of determining email address validity works in >99% cases. However, nothing is guaranteed. In a small number of cases it is possible for a mail server to report one thing on email verification and do something different on trying to deliver an email to the email address verified.

At the time of verification the mail server would have reported Yes/No, however this may have been due to an error within the target mail server and the opposite may have been true. This is rare, but it can happen. If this was a temporary error within the target mail server, please note that this result may be remembered by our system for a few hours.

For another example, say we take an email address of “” to send to. We are sending from a fictitious email address “”.” reports with status code of “OK” from the email verification API. However, when you send an email to “”, the email bounces. Further inspection of the bounced email Non Delivery Report (NDR) headers show something like the following:

Delivered-To: [email protected]
Received: by with SMTP id n6csp24867ioo;
                Sat, 6 Jun 2014 03:57:29 -0800 (PST)
X-Received: by with SMTP id 5mr1335105oie.22.1417867048986;
                Sat, 06 Jun 2014 03:57:28 -0800 (PST)
Return-Path: <>
Received: from ( [])
                by with ESMTPS id ws5si21632759obb.102.2014.
                for <[email protected]>
                (version=TLSv1.2 cipher=ECDHE-RSA-AES128-SHA bits=128/128);
                Fri, 6 Jun 2014 03:57:28 -0800 (PST)
Received-SPF: none ( does not designate permitted sender hosts) client-ip=;
           spf=none ( does not designate permitted sender hosts) smtp.mail=
Received: from ([]) by over TLS secured channel with Microsoft SMTPSVC(7.5.7601.22751);
         Fri, 6 Jun 2014 03:57:28 -0800
From: [email protected]
To: [email protected]
Date: Fri, 6 Jun 2014 03:57:28 -0800
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
X-DSNContext: 335a7efd - 4481 - 00000001 - 80040546
Message-ID: <[email protected]>
Subject: Delivery Status Notification (Failure)
Return-Path: <>
X-OriginalArrivalTime: 06 Jun 2014 11:57:28.0142 (UTC) FILETIME=[CEAD2EE0:01D0114B]

This is a MIME-formatted message.
Portions of this message may be unreadable without a MIME-capable mail program.

Content-Type: text/plain; charset=unicode-1-1-utf-7

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

           [email protected]

The email header of the NDR shows that Hotmail thinks the email address is invalid as far as sending to this address is concerned. However, Hotmail reports that the same email address is valid as far as the email verification activity performed by Email Hippo.

The discrepancy in verification results versus mail send is with the Hotmail infrastructure reporting one thing but doing the exact opposite. This behaviour occasionally (particularly from Hotmail) is seen in a small amount of cases and is attributable to internal Hotmail (or other mail services) system anomalies.

The majority (>99%) of email verification status versus mail send is consistent. However there are some edge cases caused by system faults in the mail service providers themselves. For these small number of cases, there is nothing that can be done at the email verification stage.