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.
At a topline level, reasons for non delivery 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 outcome 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. It’s normal for mail servers to keep trying to resend a soft bounced email for a relatively short period of time / number of times.