So that the reputation of your sender domain does not suffer, contacts whose e-mail address does not exist or who do not wish to receive your e-mails are automatically marked with a “Do Not Contact” (DNC).
You can find out more about this topic in this FAQ: What is a Do Not Contact and how can I remove it?
If Aivie receives feedback that the email cannot be delivered to the specified address, the contact is marked as DNC and moved to a “Bounced emails” segment created by Aivie.
There are various reasons why an email bounces. Only the so-called “hard bounces” lead to the DNC being set.
You can view the exact reason why an email is bounced in the contact history via the details:

The same applies to contacts who indicate in the e-mail program that they do not wish to receive your e-mails (any more). We call this a complaint.
Aivie monitors the number of bounces and complaints in the background. If too many emails cannot be delivered, we will pause the sending of emails via your account.