Understanding Email Bounces

Emails that can’t be delivered to a recipient for one reason or another are said to have “bounced.” Email bounces can happen for a variety of reasons, and when one occurs, a “return to sender” message usually will be sent from the recipient’s mail server to explain why the message delivery was unsuccessful.

What is the difference between hard and soft bounces?

Bounces can be put into one of two categories: hard or soft. It can help to think about hard bounces as similar to bouncing a check, because they indicate a failure that ends the transaction. Unlike bouncing a check, soft bounces aren’t necessarily a deal-breaker. Rather, they provide indications of what steps you need to take next.

What is a hard bounce?

A hard bounce is given to an email that has been rejected outright because it was determined to be invalid. This could mean several things. There may be a typo in the email address, the email address doesn’t exist, or it did—but it doesn’t anymore.

The bottom line? This address is DOA. What should you do about it? Remove it from your mailing list and add it your suppression list. (When SparkPost receives a hard bounce, we will do this for you automatically.)

What is a soft bounce?

A soft bounce is temporary—meaning, the reasons it couldn’t be delivered are fixable. Some reasons for soft bounces include mailboxes that are full, files that are too large, servers that can’t respond because they happen to be down at that very moment.

But whatever the temporary problem, a soft bounce generally means that the email address is a valid one. Your next step? In most cases, the email delivery will be re-attempted a couple times over a certain period of time before the sending server gives up. However, it’s important to keep an eye on the address and the specific bounce codes to protect your sender reputation and to ensure maximum deliverability.

When SparkPost receives a soft bounce from a receiving mail server, we log a “delay” event and retry again later; when we reach a maximum number of retries, the message will be flagged as undeliverable.

What are bounce codes?

Like everything these days, there’s a lot of data tied to email deliverability. In addition to bounce categories, each bounce will contain a code that gives the sender a brief explanation about the deliverability issue. Where exactly do these codes appear? These codes show up in the response string of an undelivered email. For example:

SparkPost (and many other email delivery services) take these codes and translate them into an easier-to-understand report in our service’s web app.

Making sense of email bounces

In theory, the difference between hard and soft bounces is straightforward. In the real world, however, the distinction often is blurrier. That’s because each ISP or inbox provider handles bounces a little differently. The best way to understand what a specific bounce means is to actually look at the bounce message or code, because those details are how the ISP will communicate their intent.

Making sense of this ambiguity is a bit of a chore with even just a few messages. If you’re sending thousands or even millions of messages at a time, that challenge is overwhelming. That’s where the benefits of SparkPost’s deliverability features really make a difference. Our Adaptive Email Network is unmatched in its ability to interpret bounces and to automate the appropriate response to ensure maximum deliverability.

How to learn from bounces to improve deliverability

Both hard and soft bounces impact your overall email deliverability rate. The best way to reduce the number of bounces is by following some key email deliverability best practices. This includes the following:

  • Be strictly opt-in. Send a confirmation email when users subscribe to your list. This way you can ensure that the user’s email is not only valid, but that they in fact want to receive your email messages.
  • Maintain good list hygiene. Purge your list regularly of invalid emails and non-responders. High bounce rates can affect your sender reputation so keeping your lists clean will go a long way to achieving higher email delivery rates. You can review and manage your bounce lists right from the dashboard of the SparkPost web app.
  • Monitor your email delivery. Track your email delivery rates by paying close attention to your bounce rates as well as your response rates. With regular monitoring, you can catch potential failures before they happen.

Learn More about Email Bounces

Read more about email bounce best practices

Dig deeper into understanding email bounces with these resources on the SparkPost Blog and elsewhere on the web.

  • The Wild West of Bounce Codes and Deliverability: A great explanation of why email bounce codes are not as simple as they seem. Our deliverability experts give practical advice for handling the idiosyncrasies of email bounce codes.
  • RFC 1893: The official Internet Engineering Task Force (IETF) standards document that defines how mail servers should respond to temporary or permanent message failures. These recommendations were further extended in RFC 5248.

Get help with email bounces in the SparkPost Support Center

Learn more about how email bounces work with the SparkPost service in the SparkPost Support Center.

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