Soft bounce – when your ESP (Email Service Provider) tries to deliver an email to an address, but the receiving mail server tells us that the email address can’t be delivered to on this occasion, but is still valid. A full inbox, server being down or message being too large are good examples of this.
Hard bounce – This refers to when your ESP tries to deliver an email to an address, but the receiving mail server tells us that the email address can’t be delivered to. It may have been closed down, an invalid email address (domain name is incorrect, not real, or recipient is unknown).
Soft bounces are temporary and these addresses can be emailed again. Hard bounces are permanent and are suppressed from future sends.