A "bounce" is when an email is sent and the server receiving the email has sent it back. There are two categories for this situation, hard bounces and soft bounces.
Hard bounces are when the email is permanently undeliverable to that email address. Causes for this include invalid email address, typos, changed address and the domain does not exist. You should not send to email addresses that get a hard bounce. These email addresses can turn into spam traps by being reactivated by the ISP to catch spammers.
Soft bounces are emails that make it to the recipient's server but gets sent back before reaching the recipient. Causes for this include a full inbox, out of office replies, the server is down, or the message is too long. You should send to these email addresses no more than three times if they keep bouncing. After a soft bounce is recorded three times for a single email address, the soft bounce turns to a hard bounce and emails should not be sent to the email address again.
Bounces will show up with every purchased email list. Make sure that the hard bounces are being removed from your list to prevent becoming blacklisted.