The delivery of some emails may fail and return an error commonly called "bounce". These errors are classified into two categories:
- SOFT bounce, which is a temporary issue
- HARD bounce, which corresponds to a permanent delivery problem
About SOFT bounces
The SOFT bounce means a temporary issue is preventing the delivery of the message. This error usually happens in the following cases :
- when the recipient email box is full
- when the email service is temporarily unavailable due to a failure
- when the size of the email exceeds the size allowed by the provider
- when the host detects a bad in reputation and temporarily refuses messages
Important: emails will go into the soft bounce blacklist only if they have not been successfully delivered to the recipient after three consecutive attempts over 3 consecutive weeks.
About HARD bounces
HARD bounce is a permanent error. This error occurs when the email address does not exist.
Formulas for 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
Are SOFT and HARD bounces recovered immediately ?
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.