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Understanding Email Deliverability, Inbox Rates and Bounce Rates

Travis Balinas
May 21, 2014

Email marketers use terms to describe what happens to an email after it is sent. While each one of these terms is different, they are all part of the same email success rate calculation.

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When you send an email out from a platform like OutboundEngine, there are a number of things that happen on the back-end to give that email the best chance of landing in someone’s inbox.

The first step in measuring the effectiveness of an email marketing newsletter is to determine how many people received it. There are three terms that go into the first set of measurements email marketers use to analyze their marketing campaigns: email deliverability, inbox rate and bounce rate.

What Is Email Deliverability?

Email deliverability is incredibly important to the success of any kind of email marketing. In short, email deliverability rate (or acceptance rate) is the success rate an email marketer has in getting an email delivered to a person’s email address. To find out the deliverability rate of your email marketing, you simply take the number of emails delivered and divide it by the number that were sent.

At OutboundEngine, we have a 99% deliverability rate for the emails we send for our customers. What’s important to know now is that there are many safeguards in place to prevent people from blasting or spamming large email lists. Protecting our reputation and the reputation of our customers ensures high deliverability while keeping us in good standing with our IP address.

What Are Inbox Rates?

A good deliverability rate ensures that various email servers accept the emails we send. The inbox rate (or inboxing rate) is the measure of how many of those emails are sent to the inbox as opposed to the spam folder or bounced.

The simple way to calculate this is the number of emails that reach the inbox divided by the number of emails that were sent. This number does not include the emails that are sent to the spam or bulk folder.

High deliverability is important, but getting those delivered emails to appear in the inbox is imperative. Our goal and current inboxing rate is 100%. We calculate this number through third-party partners who measure the top 25 email domains out there (i.e., Gmail, Yahoo, Comcast, Hotmail, AOL, etc.). We cannot measure across every email that we send because we don’t own all of the inboxes out there, but through our partner network, we are able to know if our inboxing rate ever dips below 100%.

What Are Bounce Rates?

The bounce rate metric is defined as the number of emails that are rejected by the receiving server. While the deliverability rate (or acceptance rate) counts the number of emails that the receiving server does accept, the bounce rate counts the emails that are returned to you. High email bounce rates will negatively impact the sender so it’s important to keep this rate low.

Email bounces fall into two categories: hard bounce and soft bounce.

A hard bounce is the more severe of the two bounce types and means that the email is completely undeliverable. If this bounce occurs, we won’t attempt delivery to that address in the future for our customers. If an email is hard bounced, it’s typically for one of three reasons:

  • Email address/recipient does not exist
  • Domain the email was set to does not exist
  • The receiving server has permanently blocked delivery

A soft bounce is handled a little differently than a hard bounce because it indicates a temporary issue. After attempting delivery to an email address over a few days, it will finally be marked as a soft bounce. Soft bounces can occur for any number of reasons and a few are more common than others:

  • Server or mailbox is full
  • The server handling the recipient email is down
  • Email is too large to be delivered

It goes without a saying that having a low bounce rate is important and at OutboundEngine, our customers have a less than 1% bounce rate. But even when a bounce does happen, even if the percentage is less than 1%, it’s still important to analyze why emails were unable to be delivered. That’s why we’re constantly monitoring and adapting to changes in the rules that govern email delivery to keep our bounce rate below 1%.

As email marketing continues to evolve, the only way to keep up is to continually adapt. However, without monitoring and measuring the success of your email campaigns, you won’t know or understand how to modify your approach if things go south. Email marketing is complex, but the first step is getting your email delivered. If you can’t do that, then there’s no point in sending an email.


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