Bounces can be either hard bounces (permanent) or soft bounces (temporary)

Hard Bounces
A hard bounce is where the email address is not valid, or the email account doesn’t exist.  Our system automatically handles these bounces for you and marks the contact as invalid in your database.  We do not send emails to these addresses (and do not charge you for them either).  You can edit the email address associated to an invalid contact and effectively reactivate it.

Soft Bounces
A soft bounce is where the email couldn’t be delivered to a valid, existing email account. The usual reasons for that is when their server is unavailable, or their mailbox is full.  The contact remains in your account as active – we will continue to send mails to that address.