When your email has not been successfully delivered and the reason reported back to us is that the email has bounced, that email appears on YAPI Email Problem Report. 

Any bounced email will not be delivered to by YAPI in any future mailings. Email must be explicitly  removed from Email Problem Report to allow future mailing.

There are two main categories of bounced emails:

soft bounce is an email message that gets as far as the recipient’s mail server but is bounced back undelivered before it gets to the intended recipient. A soft bounce might occur because the recipient’s mailbox is full, the server is down or swamped with messages, or the message is too large.

hard bounce is an email message that has been returned to the sender and ispermanently undeliverable. Causes include invalid addresses  or the email recipient’s mail server has blocked your email.

More details are here.