Skip to main content

What are SOFT and HARD bounces?

Splio Knowledge BaseMarketing AutomationSettings Troubleshooting

Definitions

The delivery of some emails and SMS may fail and return an error commonly called "bounce". These errors are classified into two categories:

  • SOFT bounce, meaning a temporary issue.

  • HARD bounce, meaning a permanent delivery problem.

To understand the difference between soft and hard bounces depending on the canal you are using, you can read the following table.

SOFT bounces

HARD bounces

Emails

  • the recipient email box is full

  • the email service is temporarily unavailable due to a failure

  • the size of the email exceeds the size allowed by the provider

  • the host detects a bad reputation and temporarily refuses messages

  • the email address does not exist

SMS

  • the mobile cannot be reached because it is located outside a coverage area, positioned on airplane mode, or switched off

  • the mobile is located in an area insufficiently covered by the operator network

  • the target mobile is experiencing a hardware or software malfunction

  • the operator being involved is experiencing a technical breakdown

  • the phone number is invalid, or unassigned

  • the operators are facing all kinds of malfunctions, which they report

Calculating bounce rates

The performance evaluation of the targeting and the sending can be done based on the SOFT and HARD bounce rates. The bounce rates are calculated using the following formula:

  • SOFT bounce rate = number of SOFT bounce emails / number of emails sent

  • HARD bounce rate = number of HARD bounce emails / number of emails sent

Understanding bounce recovery (emails only)

Almost all soft or hard bounces are obtained synchronously, which means during the SMTP transaction the email server returns directly a confirmation or an error if the recipient exists or if the message has been accepted or not.


However some bounces are recovered asynchronously, in this case, they are obtained through the return address set for sending. In rare cases, these returns can be sent and thus recovered several hours or even several days after sending the message.