A soft bounce occurs when an email reaches the recipient, but fails to deliver resulting in a "bounce" back. A soft bounce implies that the delivery issue is temporary and can be a result of many different causes, some of which include:
Email server is down.
Recipient's inbox is full.
Email is not configured correctly.
Mailbox is currently inactive
A hard bounce occurs when an email is permanently rejected by the recipient. This can be a result of several different causes, some of which include:
Recipient's email is invalid.
Recipient's domain does not exist.
Recipient's email server has blocked your emails.
How does this directly affect you or your campaign?
Email marketing is impacted by many various metrics, two of which are email deliverability rate (acceptance rate) and bounce rate (failed rate). For this reason, it is important to keep bounce rates as low as possible to increase the likelihood that your campaigns are successful.