You can capture campaign emails that aren't delivered to user mailboxes.
Go to Phish Threat > Settings > Bounced Mailboxes to manage your bounced mailboxes.
If a campaign email isn't delivered to a user mailbox, the email is added to this list of bounced mailboxes. The page contains the email ID and the campaign name for which the email delivery failed. It also contains failure details such as the DNS code and the failure SMTP error. You can filter the list by username, email address, campaign name, or bounce type.
Future campaigns will not be sent to users in the bounced mailboxes until the issue is resolved and the users are removed from the list.