This article explains the differences between hard and soft bounces.
When you send emails you will sometimes receive a response from the server or a report from your email service provider telling you that your email(s) have not been delivered and giving you a reason why.
Reasons for non-delivery of emails fall into two categories, hard or soft bounces.
If emails hard bounce, it is a permanent problem. You should delete hard bounce emails from your list. Repeatedly trying to send to them, will not change the hard bounce and could affect your sender reputation. Reasons for hard bounces include emails that are not valid and mail servers or domains that don’t exist.
Emails that soft bounce have experienced a temporary situation that has prevented them from being delivered. Common reasons for soft bounces include mailboxes being full or the file size of the email being too large, for the incoming mail server. It’s normal for mail servers to keep trying to resend a soft bounced email, for a short time or after several retries.