Introduction to Email Bounces

When Email Notifications Become Email Bounces

Bounced and undelivered email notifications can be a particularly vexing challenge for teams building SaaS products. In this article, you will learn:

Why email notifications sometimes bounce

Bounced and undelivered email notifications are a fact of life, but they can be a particularly vexing challenge for teams building SaaS products.

A mistyped or invalid email address associated with a user’s profile or account is a common reason. Avoiding that problem is why email verifications are very often the first step in creating a user account.

However, 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. SaaS teams can use those bounces to identify the cause of (and perhaps resolve) problems that prevent users from seeing critical email notifications.

Email bounces are not all alike

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.

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? Stop sending notifications to it and alert your user to update their account.

In contrast, 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. 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.

Hard Bounce

An email that has been rejected outright because is was determined to be invalid. Examples include:

  • Invalid recipients
  • Blocked attachments
  • Denied relay requests

Soft Bounce

An email that couldn’t be delivered for reasons that are temporary and fixable. Examples include:

  • DNS failures
  • Full mailboxes
  • Vacation/Out of Office auto-replies

These are just a few common examples of the many types of email bounces that exist. Email services classify bounces in different ways, depending upon the requirements of their systems. For one typical example, see SparkPost’s list of bounce classification codes.

Bounces must be managed

The raw processing of bounces happens behind the scenes with your app’s email infrastructure. However, it’s important that your app monitors bounces to ensure your users aren’t missing critical notifications.

Proactive management of bounces also important for maintaining a good reputation with mailbox providers like Gmail and Hotmail so that your app’s emails have the best chance of landing in your users’ inboxes (sometimes called email deliverability).

Bounces contain actionable data

Email messages and bounces are rich with data. That data is one reason email is a very effective notification technique for SaaS apps. In the case of bounces, each message will contain a code that gives the sender a brief explanation about why the email was not delivered.

Where exactly do these codes appear? These codes show up in the response string of an undelivered email. In this example, a mail server sent a bounce message indicating it was unable to look up DNS entry for the domain of the recipient address:

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.

Making sense of this ambiguity is challenging with even just a few messages. Because most SaaS applications send hundreds, thousands, or even millions of messages a day, that challenge can seem overwhelming.

Nonetheless, bounces embody a wealth of actionable data that SaaS teams can use to understand and improve their user experience and engagement. Good analytics and data integration with email delivery infrastructure is an important part of implementing email notifications.

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