Bounce Basics

Here at MNB, we get a lot of questions about bounces and bounce rates. In an effort to clear things up, we thought we’d take a quick look at some of the basic information you need to know about bounces, and explore how they affect your email marketing campaign. We’ve got a lot to cover, so let’s get right to it.

What is a Bounce?

A bounce (or bounce message) is a non-delivery report issued by a recipient email server after a send (usually) from an email service provider. In short, it means that your email newsletter didn’t reach its destination. Bounces appear in two different forms: hard and soft, but we’ll get back to that in just a minute. First let’s take a look at why a bounce occurs.

Why Does an Email Bounce?

Usually, a bounce is the result of something like a full inbox, incorrect domain, or deleted address. These issues force a non-delivery and result in a bounce report returned to the sending client.

However, bounces can also stem from recipient host restrictions.

Every email domain server host (like or has an established set of parameters that email service providers must meet in order to successfully deliver a message to a recipient inbox. Those parameters include very specific caps for sender statistics like reputation, spam reports, bounce history and even recent sending history.

If a recipient server’s particular maxims are exceeded in any of those categories, the domain may determine the message undeliverable, and issue a bounce. Unfortunately, those specific guidelines are unique to each domain (although, they’re usually fairly similar to industry norms). And, what’s worse is that they’re not always consistent throughout the year. Many host servers will adjust their protocols during those parts of the year when inbound email is higher than average.

Finally, host domains don’t make their guidelines public as that information could help spammers penetrate their security protocols. Add all that up, and it’s easy to see why determining an exact formula for how to avoid bounces isn’t easy – for senders, or for sending clients (like us).

Bounce Types

Once a host domain determines that an incoming email will be reported as a non-delivery, it is required (adherent to Internet law) to attach a six digit code to the report, identifying it as a hard (code starts with a 5) or soft bounce (code starts with a 4). Hard and soft bounce designations are made available to the sender, offering a little insight into why the email wasn’t delivered.

So, what separates a hard bounce from a soft bounce?

Well, a hard bounce is issued in the event of a permanent problem with deliverability. This can include deleted addresses, incorrect domains or a domain established block. MyNewsletterBuilder reads into every hard bounce to determine whether or not it’s legitimate, and if it’s really permanent. If the system validates the permanent bounce, it will remove the address from your subscriber list automatically. We don’t rely on a simple 5xx code to determine a hard bounce as some domain admins report codes improperly. This helps you conserve valuable list space, rather than wasting it on dead addresses.

A soft bounce, on the other hand, is the result of a temporary issue with deliverability. That can include a domain service interruption, a full inbox or a host identified problem with content or issuer’s sending reputation. Soft bounces can occasionally overlap with spam filter redirects, but not always (it usually depends on server protocol).

After receiving an initial soft bounce report, MyNewsletterBuilder’s servers will repeat the send attempt over a period of 24 hours to try and complete delivery. If, after the sending period is over, the recipient host is still issuing soft bounce reports, MNB will record the send as a soft bounce in your report data.

We’re Not Done Yet

So, what do you do with bounced emails? What should your bounce report look like? How can you reduce the number of bounces you’re reporting in the future?

We’ll get to all those questions and more in two weeks when we return with part 2 of Bounce Basics blog breport – err, report.