When email is sent to a contact and the remote mail server returns the email to us, there are two types of bounces, hard and soft.

Soft bounce:

When an email bounces on a temporary basis, such as "mailbox full", "user over quota", "server unavailable", etc. Savvy Sender automatically unsubscribes contacts from a list if their email address has bounced up to seven (7) times; however, the count is reset if a contact opens an email or clicks a link.

Hard bounce:

When an email bounces on a permanent basis, such as if an email address no longer exists or is invalid. This will cause the contact to be marked as “Do Not Email”, which blocks all future emails regardless of whether the contact is subscribed to a list.

In addition to hard and soft bounces, emails can result in an error if a domain no longer exists or a domain does not have a proper MX record.