Soft bounces are temporary errors that prevent us delivering your message to a recipient. This can be related to an Internet Service Provider experiencing difficulties with its infrastructure, for instance. Hard bounces are permanent errors, such as an invalid address.

It is not always easy to know if a bounce is soft or hard. Sometimes a recipient delivery box is full for a while or a service is going through extended technical challenges. When we don't know, we keep trying until we are sure that a bounce is hard. Our statistical bounce engine has become really good at this game over time and we rarely miscategorize messages as "Hard bounce".

You might see the same address bouncing several days in a row. That's precisely because we are really persistent before categorizing an address as “Hard bounce”.