Difference between "hard bounce" and "soft bounce"?

Great question!

A hard bounce is a permanent rejection of an email.  In almost all cases, an email will be permanently rejected by an inbound server if there if the recipient email address does not exist.  A large number of hard bounces occurring over a short period of time can definitely result in reputation damage.  For this reason, Customer.io will automatically suppress any further attempts to send email to an address that has hard bounced in the past to offset these negative impacts.

A soft bounce is a temporary rejection.  There are a wide variety of reasons that an email may be temporarily rejected including (but not limited to): an inbox is full or over quota, the domain or IP has been rate limited (otherwise known as throttling or, slowing down the delivery rate for a given message), the IP or domain is on a blocklist, or other similar scenarios. 

In some cases, our system will attempt to re-send those temporarily rejected messages.  In others, no retry attempt will be made.  Either way, the email address will not be suppressed from future sending attempts.  A blocklist is an example of a case where we won’t re-try a message immediately, but sending is still allowed in the future.

With goal of analyzing your lists and keeping them healthy in mind, we recommend applying all of our suggested List Hygiene best practices outlined in our documentation here.  By focusing on your engaged recipients and adopting a list-removal criteria, you can be sure that you are focusing only on valid recipients who consistently engage with the emails you are sending.