I have been on an email cleanse lately, so going back and deleting old emails that there is no reason to keep. Over the years I have been quite good at labeling anything that is keep-worthy, but a few can slip through the cracks. Here is a simple search you can run inside your gmail box that shows all the emails you can look through and delete.
Search for this to find anything that doesn’t have a label, isn’t in trash, your inbox nor drafts. The new Gmail search should return 100 results, so look through and see any you may have missed to label.
Search box: “has:nouserlabels -in:trash -in:inbox -in:draft”
-Then select all by clicking on the square in the top left of the area and select all. Uncheck any you want to keep, then click the trash icon.
The ones you un-checked will show up at the top of the list now, read them and set a label on the email/chain if you want to keep them.
-A much faster way to do this is to enable keyboard shortcuts in the settings area. Then you can press * then a so “*a” without the quotes and this will select all. Then press # to trash them.
Google will remove things from your trash after 30 days, so you have a little bit of a window if you deleted something and need it again.