When you deploy an email there will be some addresses that the system is unable to deliver to. When the recipient email address "returns the email to sender" it is referred to as a bounced email. the marketing system will keep a record of these returned emails. There are two categories of "bounced" emails: a hard bounce and a soft bounce.

A hard bounce means the marketing system attempted to deliver the email, but the email address does not exist or there is some other problem that cannot be resolved. Hard bounces are placed on the "invalid" list and the email system will not attempt to deliver to that email address again.

A soft bounce means that the persons mailbox was full, or the server is down, or there is some other problem that may be temporary. The email address may be valid, but it is not working at this time. These soft bounces are put into a kind of "purgatory" for a few days. During that time you cannot send an email to that address because the system is giving it an opportunity to resolve the deliverability issue. If the email address continues to bounce in future mailings it will eventually be marked as permanently invalid.