If you have found that contacts are being suppressed as bounces, but are unsure as to when or why they bounced, there is a report that shows every bounce that has occurred from your Instance's mailings.

This can be found under Reports->Email->Email Global Undeliverables:

To navigate to the report, see below:

 

Once in the Reports sections, see below for the report setup:

 

You can set the start and end dates of the report, or use the 'Last' field to designate a certain number of days or months you wish to the report to fetch.