Bounced means the receiving mail server permanently refused the address (non-existent mailbox, blocked domain). Spam complaints are recipients who pressed Report spam. Both are reported back by the email provider and shown on the campaign's Stats overview as Unsubscribed / Spam complaints / Bounced, calculated as a percentage of everyone the campaign was sent to. The provider automatically stops sending to bounced and complaining addresses in the future.

To keep these low: only upload contacts who opted in, remove old or purchased lists, keep the Footer module with a visible unsubscribe link in every design, make sure onboarding's domain and link validation are complete (authenticated domains land in the inbox more often), and avoid sending the same audience too frequently. High complaint rates can lead the provider to suspend the sending account.