Throttling - Definition

Throttling is the intentional process of slowing by an internet service, to regulate network traffic and minimize congestion.


Email Hippo aims to provide you with the most accurate results as quickly as possible. Technically, our system can verify one million emails in 1 hour, but email servers will often view email validation attempts as an attack because,  at first glance, they look very similar. 

The Problem - Same Domain Queries

Many email services and systems have built-in defence systems to prevent both spam and Distributed Denial of Service (DDoS). These preventative measures are there to ensure continuity of service and reduction in spam.

For DDoS protection, mail servers will monitor the rate of work (e.g. connections and protocol handshakes) from IP addresses. If the rate of work from a given IP address exceeds certain thresholds, then the mail server will block further work from the connecting IP address. This block is otherwise known as an “IP Block”.

The consequences of an IP block is that further email verification work cannot be performed. Results obtained during an IP block event will return as either ‘Bad’ or ‘Unverificable’.

IP Blocks can be temporary (or transient) or permanent.

Continually querying the same domain with multiple permutations of [email protected] is almost guaranteed to create IP Blocks at some point in time.

These proprietary IP Blocks are outside of the scope of control of Email Hippo.

Email Hippo and IP Addresses

Email Hippo is aware of some of the constraints of remote mail systems in terms of throttling based on IP address. Hence it has in place predictive IP address rotation schedules and, for bulk systems, there are adaptive throttling controls in place.

However, for real-time systems (e.g. our API), the processing rate (and any appropriate throttling) is completely within the control of our customers. It is, therefore, the real-time systems that can be vulnerable to the effects of IP Blocks.

How to Mitigate IP Blocks and Improve Results and deliverability, a number of factors can affect validation results as far as “same domain queries” are concerned:

  1. Continuously validating the same domain in a short amount of time. 

  2. Continuing to validate a single email address because you don't agree with the result. 

  3. Uploading a list of a single domain to be validated.  

To a mail server doing these things can look like you are trying to gain force entry to the mail server and are looking for a point of weakness, resulting in the mail server blocking the IP address of the validation process and involving the system to rotate to a new IP address. By slowing down your validation attempts not only will you receive better results but you can also apply the same rules to your marketing campaigns. 

Sending out emails in quick succession one after the other to the same domain can result in your email service provider (ESP) also being marked as spam. 

Throttling your validations and marketing emails will not only get you better results but will also reduce bouncebacks as your emails won't be classed as spam so often.