If I delete a project or opportunity, will that also delete associated emails, etc. that have been stored via DMI?

Hello Greg,
Since those emails can be linked to other objects in the database (ie Contacts, Orgs, other Opps, etc), deleting the Opp or Project does not delete the items that are linked to it. If you *do* want to delete those emails, I would suggest clicking on the Opp you wish to delete, click the Activity button, highlight the emails, delete the emails, then delete the Opp.
This being said, you may want to delete the Opp's linked Tasks, Appointments, Notes, etc if you want to erase all existence of the Opp and it's related activities.

Hope this helps,
Cynthia