Classifying email bounces and managing them in your audiences

It’s a common adage that a few topics should be avoided in conversation among friends, as those topics can carry the potential for messy disagreements. These typically include subjects like politics, religion, personal finances, and – of course – email bounce handling. 

In the world of email deliverability, the optimal process for identifying, analyzing, and potentially suppressing bounced recipients can be as controversial a topic as they come. 

Each provider is likely to have a distinct methodology for handling bounced addresses, which can be a bit confusing for marketers who simply want to maximize their reach while minimizing their risk. 

To help alleviate some of that confusion, let’s dive into how MessageGears classifies email bounces and our recommendations around how to manage them in your audiences. 

What does an email bounce really mean?

A bounce occurs when a sent email message can’t be successfully delivered to the intended recipient. The cause of the bounce can fall into one of many categories, but they’re often lumped into two main groups. These can sometimes be called “hard” or “soft” bounces, although these terms may be a bit vague.

We typically use the terms “temporary bounce” and “permanent bounce” to refer to these major groupings, and we prefer to speak more specifically to the cause of the bounce when possible. On our Support site, we’ve compiled a comprehensive list of the codes we use to categorize each bounce type, along with a deeper dive into the data used for the categorization. 

Temporary bounces

For now, let’s say that a temporary bounce (often called tempfail or deferral) means we’ll keep trying to deliver the message for up to 24 hours. These email bounces are identified by a response code of the format 4xx. They can include temporary issues like server errors or throttling of mail by the receiving server. Essentially, these bounces are the receiver telling the sender to “try again later.”

If, after the 24-hour retry period, the message continues to be rejected, our servers will stop all delivery attempts and log a bounce for that recipient and message.

Permanent bounces

In contrast, a permanent bounce is the receiver’s way of telling the sender to immediately stop trying to deliver the email message in question. These bounces include a response code of the format 5xx and can be caused by invalid recipients, bad domains, full mailboxes, and even spam blocks. When our mail servers receive this type of response, they stop all attempts to deliver that specific message and log the attempt as a bounce. 

In some cases, it’s necessary to take action beyond stopping the delivery attempt for a single message. The email bounce response received might indicate the recipient address has a more systemic issue that will prevent mail from being delivered in the future. In these cases, it may be necessary to suppress or remove the address from your mailable audience. 

When should you suppress a bounced address? 

The primary reason to suppress bounced recipients is a response that indicates the address doesn’t exist, or some other persistent problem that’s not likely to be resolved. Knowing when to make that choice isn’t always easy: continuing to mail addresses that bounce is likely to damage your sender reputation, while removing addresses that are still valid will needlessly reduce your marketable audience. 

In early 2020, our team researched billions of email bounces to understand how likely bounced addresses were to engage with a message going forward (read our recap and analysis of the data on how bounces impact super senders and how you should follow up a mailbox full bounce). 

Based on our analysis, we made some general recommendations on suppression rules for each type of email bounce. Since that time, we’ve reviewed additional data and made slight adjustments to our recommendations. Here is our most updated guidance for each type of bounce:

Invalid Recipient 

These bounces indicate the intended recipient does not exist, and the associated address should always be suppressed. Some research has shown that 1-2% of those addresses rejected as Invalid Recipient may end up with a successful delivery in the future, but it’s our position that the risk to reputation is too great for such a small potential return. We recommend immediate suppression after a single occurrence. 

DNS Failure 

These generally warrant suppression as well, but a bit of caution is advised. DNS failures can occur due to general internet outages, specific server issues, or even unreliable DNS resolvers. As such, we typically suggest setting a rule to suppress after 2-3 consecutive occurrences. 

Mailbox Full 

A full mailbox may be a sign the address is abandoned, or it may be a purely temporary condition. We suggest suppressing these bounces only after multiple occurrences, with the number of occurrences depending on your mailing pattern. Is it possible someone would receive three messages in a single day (including transactional/triggered)? 

If so, we’d advise starting with a higher value – maybe five, seven, or 10 – since data indicates there’s about a one in three chance they will engage with an email moving forward. 

If recipients get just a message or two a week, you can start with a lower total like two to four occurrences. It may also be a good idea to specify the number of times these bounces occurred in a set number of days; e.g. seven bounces within 14 days. 

Spam Block / Spam Content 

Bounces caused by spam filters present unique challenges when deciding whether to suppress. The address is most likely still good, but the mail is blocked for content or reputation. The first course of action here is to work on addressing the underlying cause for the bounce. 

This may include reaching out to the administrators of the recipient domain (for reputation-based blocks) or testing content to find the offending elements (for content-specific blocks). 

We typically recommend suppressing these only if the spam block is persistent (multiple occurrences in a set number of days without a delivery) and only after attempts to resolve the issue causing the blocks. 

Generic Bounce / Undetermined

The lack of detail provided for these email bounces can make it tricky to apply suppression rules. In many cases, these are spam-related bounces, but the bounce response returned by the receiving server does not give enough detail to properly categorize it. 

Reading the text of the bounce response (also known as the Delivery Status Notification or DSN) may provide some insight, although many bounces include very vague text or none at all.

If you’ve recently made any changes to your mailing program – new ESP, new source of contacts, new domain, etc. – it’s a good idea to review each set of bounces on a case-by-case basis when possible. If your program is more stable and you’ve made no major changes, it may be possible to set suppression rules for these types of bounces. 

We advise allowing multiple consecutive occurrences with no successful deliveries, with the number of bounces required dependent on your mailing frequency (see our recommendations for a Mailbox Full bounce response above). 

Managing your email bounces 

While the optimal process for identifying, analyzing, and potentially suppressing bounced recipients may leave some room for debate, we hope this blog helps you feel more confident classifying bounces and managing them in your audiences. 

Email bounces are an important indicator of the health of your email list, so it’s important to feel in control of your list and the process you use to manage the bounces that occur along the way.