A bounce is an email whose delivery was rejected by the recipient email server. The reason an email is rejected for delivery determines if that bounce is considered "hard" or "soft". You'll notice these in the Activity Feed and email reports.
A hard bounce indicates a permanent reason an email cannot be delivered. Causes include invalid addresses (domain name doesn't exist, typos, changed address, etc.) or the recipient's mail server has blocked delivery.
A soft bounce gets as far as the recipient's mail server (it recognizes the address as valid) but is bounced back undelivered before it gets to the intended recipient. A soft bounce might occur because the recipient's mailbox is full, the server is down or swamped with messages, or the message is too large.
Sometimes when we attempt to deliver mail we receive what's called a "deferral" from the recipient's server. Learn more about deferrals.
What happens after a bounce occurs?
We automatically disable recipients who hard bounce from receiving future emails. We do this to ensure your email deliverability rate remains high. Continuing to try to send to someone with a known bad address will harm your sender reputation, which affects how likely your email is to end up in someone's inbox.
Can I re-enable email permission for a contact that hard bounced, so they can receive email again?
Yes, however please do so with caution, as continually emailing people who have previously bounced will affect your sender reputation. This should generally only be done in the case where you has an incorrect email address for a contact.
How are out of office replies treated?
We don't treat out of office responses any differently. They show as "delivered".
Note that some email distribution lists (e.g. Office 365) don't receive auto replies.