When sending a Maiilburst marketing campaign email, it might not get to the subscriber for many reasons.
One of the most important is when the message gets bounced. There are two types of bounces: soft and hard.
The most common reason for a soft-bounce is mailbox full. Other reasons for soft bounces are: mailbox provider service down, email rejected, email size too large.
Hard bounces happen when the mailbox is not available, meaning the user does not exist. When a contact hard bounces or complaints, they are not sent emails again. The contact remains in the Subscribers list, but they can be deleted manually to avoid having old email addresses that are not being used.