Why do emails keep getting placed on the bounce list?

Why do emails get placed on the bounce list. This is the second time it has happened and I don't notice until someone complains about not getting emails. REALLY FRUSTRATING!

The email address will be listed in the bounce list when the recipient refuses or rejects the incoming emails from us. It is possible that your email service provider is blocking our emails.

Our servers will try several times to deliver the messages for a given time-frame, but if the delivery constantly fails, it will mark it as "old", "rejected" or similar, and it will store your recipient email address into our bounce list. Please check this guide: How-to-setup-email-alerts-to-prevent-email-bouncing-related-issues

Hope you understand. Thank you.