A "soft bounce" and a "hard bounce" are terms used to describe the delivery status of an email message.
Soft bounce: A soft bounce occurs when an email message is temporarily undeliverable due to a temporary issue, such as a full inbox or a temporary network issue. The email server will attempt to deliver the message again at a later time.
Hard bounce: A hard bounce occurs when an email message is permanently undeliverable due to a permanent issue, such as a non-existent email address or a blocked domain. The email server will not attempt to deliver the message again.
It is important to monitor the bounce rate of your email campaigns and address any hard bounces, as sending emails to invalid addresses can harm your sender reputation and negatively impact the deliverability of future emails.