You like it or not, when you start sending emails, not all your addresses will be valid and when sending emails to invalid email addresses, these emails will return to you in order to notify you that the given email address is not valid anymore.
In order for MailWizz to be able to catch these emails and take proper actions against them, you need to use a feature called bounce servers.
Bounce servers are actually regular email boxes that are used by MailWizz in order to catch the returning emails and take proper actions. After creating a bounce server, make sure you associate it with a delivery server, otherwise bounce processing will not work.
Please note that delivery servers of type web api do not need bounce servers. The bounces are processed automatically through webhooks.