How ESPs Manage Hard Bounces

An OpenMoves customer recently telephoned and expressed concern that our hard bounce reporting was inaccurate. Having only just become our customer, and having come from another Email Service Provider (ESP), they explained that the same list they’d used previously was now reporting a number of hard bounces and wouldn’t allow email campaigns to be sent to some of the listed addresses.

Equally concerned, we started to investigate. We asked our customer to supply us with a small sample of names that their former ESP had classed as “’delivered” but we had hard bounced.

We discovered that not all ESPs handle hard bounces in the same way.

What Does This Mean?
To understand the process of email delivery, we’ll take a few steps back.

When any email server (belonging to a company or an ESP) sends a message – for example – to, the first step the server takes is to discover which machine serves email for the OpenMoves domain (

Once this machine is identified, the next step is to connect and exchange mail using a simple chat process. In the following example, is mailing a note to Here’s what it might look like:

In the above example, if Rose did not exist, some client servers would instantly reply with an error code that tells the sending server that Rose is a hard bouncer and does not exist. That’s what OpenMoves does.

However, some mail servers do not instantly reply and first accept all mail that is sent. In this case, once the email address is labeled as a hard bounce and the error code is pinged back, the initial connection between server and client is already gone.

OpenMoves takes every effort to process bounced emails no matter how long after they arrive and then mark them appropriately. This takes considerable resources to manage and it is understandable why some ESP’s process bounced reporting differently.

We believe that our customers want the most precise stats, so there really is no alternative. Further, the last thing that email providers like Gmail or Hotmail want is an ESP that continually sends messages to people who do not exist. Finally, you pay based on the amount of emails you send, so why would you want to send to email addresses that will never receive your message?

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