The delivery of some SMS may fail and return an error commonly called "bounce". These errors are classified into two categories:
• SOFT bounces corresponding to a temporary delivery problem
• HARD bounces, which correspond to a permanent delivery problem
The SMS bounces have no impact on email deliverability.
SOFT bounces imply a temporary problem preventing the delivery of the message. Generally it happens in the following cases:
- 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
A contact is added to the Soft bounces blacklist if three consecutive soft bounces happen in 3 weeks.
In a wider context of SMS deliverability, the notion of HARD bounce is more nuanced. In this context, HARD bounces can indeed report a permanent error, such as an invalid or unassigned number. These errors can be found also in several malfunctions of all the kinds reported by operators.
Formulas to calculate bounce rates
The performance evaluation of the sending can be done based on the SOFT and HARD bounce rates. These rates are being calculated using the following formula:
- SOFT bounce rate = number of SMS SOFT bounces / number of sent SMS
- HARD bounce rate = number of SMS HARD bounces / number of sent SMS