Bounce rate is the share of emails you send that fail to reach the recipient's inbox and are returned undelivered. A "hard bounce" means the address doesn't exist; a "soft bounce" is a temporary failure like a full mailbox.
Bounce rate matters enormously in cold outreach because it directly harms email deliverability. Mailbox providers interpret a high bounce rate as a sign of spammy, low-quality sending, and they respond by routing more of your future mail to the spam filter — or blocking it entirely. This damages your sender reputation even with the recipients whose addresses were valid.
The defense is email verification before sending. Running guessed or scraped addresses through a verifier and discarding the ones that don't confirm keeps your bounce rate low and your deliverability healthy. This is especially important with pattern-based email guessing, where some guesses will inevitably be wrong.