When an email cannot be delivered to a recipient, it's recorded as a bounce. There are two types of bounces:
A temporary issue prevented the email from being delivered. This could mean that the recipient's mailbox is full, their email provider is having issues, or your email was too large. Sometimes it just means an out of office response was received.
A permanent issue prevented the email from being delivered. This could mean that the email address doesn't exist, or the email provider has blocked delivery of your email.