What’s in a bounce?

Yesterday, a friend found a terrible post about bounces.  We talk about looking at your bounces and understanding what they mean in regards to your list health, but what are bounces and why should you care?

There are a few types of bounces.  The two main types are hard bounces and soft bounces.  Bounces are based off the SMTP reply codes that we receive back from your recipients' mail servers.  When we get the reply code, we bucket that response into a category.  Our bounce processing actually goes a bit farther than just looking at the SMTP response code and examines the text of the response for key words to make sure that we're putting the bounce in the right bucket.

Hard bounces are when the email that you sent will never be delivered.  These are typically the 5xx series codes, such as Invalid users.  If the email address does not exist, you will see that show up as a hard bounce.  The receiving mail server just told us we're not going to be able to send that email to them.  Sometimes this just means that the mailbox is full so we should try again with the next send.  Those have different numbers for the xxs.  If the mailbox doesn't exist, we don't try again with the next email you send.  If the mailbox if full, we do.  Some spamblocks show up as 5xx.  AOL's most common blocks are 5xx.

Soft bounces are when the email that you sent cannot be delivered right now, but we should keep trying for up to 72 hours.  These are typically the 4xx series codes, such as Service unavailable.  This may mean that the receiving mail server is currently overwhelmed or the mailbox is currently unavailable.  Some spamblocks show up as 4xx.  Yahoo's most common blocks are 4xx. 

When a bounce is processed after the SMTP transaction, that is called a Non-Delivery Report (NDR) or Delivery Status Notification (DSN).  These can also be hard or soft bounces based on the SMTP code.  The only difference is that they do not occur immediately during the send.   

A well-maintained list should have fewer than 2% total bounces, but most ISPs don't tend to get twitchy until you have more than 10% bounces.  Bounces of more than 10% indicate an old list or blocking problems.  You should always see more hard bounces than soft bounces.  If you see more soft bounces, then you're probably seeing blocking. 

It is possible, even on very large lists, to have bounces under 0.5% on a regular basis but you need to be willing to clean the dead wood out of your list.  When you do that, you're also cleaning out the people who aren't responding and interacting with you.  It's that kind of list maintainence that gets you into the inbox and keeps you there. 

  • Ken Fox

    I prefer the term "rejected" over "bounced" because bounce is often casually used by people who have no idea how SMTP works–too many people assume bounce and non-delivery are the same thing. Do you ever contact mail providers to find out what the true delivery rate is? Do they trust you enough to reveal accurate numbers? Async spam filters for example must accept tons of mail that goes straight to the trash. I’m sure rejection rates correlate to non-delivery, but it would be really useful to see non-delivery rates instead of hard and soft bounce rates.