Hi Chris,
In the Analytics section there is a report called Email performance report.
http://community.marketo.com/MarketoArticle?id=kA050000000KypPCASI would then pick some emails that you would consider having a poor deilvery rate and create some smart lists using the Email bounced and email hard bounced filters.
In the results page you might notice something like a lot of bad email address.
You can also open up one of the leads go to their activity log and search for the bounce, clicking on the activity ID will give you the specific bounce message. Check multiple leads to see if the same bounce message is always occuring or if there are avariety of bounce messages.
Catha