A prospect is automatically removed from future email sends when they either hard bounce or soft bounce five times. A hard bounce occurs when an email is permanently rejected by the recipient’s server, usually because the email address is invalid, non-existent, or blocked. A soft bounce occurs when an email is temporarily rejected by the recipient’s server, usually because the mailbox is full, the server is down, or the message is too large. A hard bounce indicates that the email address is no longer valid and should be removed from your list. A soft bounce indicates that the email address may still be valid, but there is a temporary issue that prevents the delivery. However, after five soft bounces, Marketing Cloud Account Engagement will mark the prospect as undeliverable and stop sending emails to them