What is bounce rate?

A bounce is a message the receiving server refused. A hard bounce means the address does not exist; a soft bounce means it exists but could not take the message now. Only the first says anything about your data quality.

Bounce rate matters out of proportion to its size because mailbox providers use it to judge whether you are a sender worth accepting. A few percent is tolerated. Past a threshold that nobody publishes exactly, delivery degrades for everything you send, including mail to people who want it.

This is why verification is not a nicety. A list that has not been checked will contain addresses that were correct a year ago, and sending to all of them to find out which still work is the most expensive possible way to learn it.