"Gmail saves everything you have deleted for up to 30 days," says John Carter. He continues, "You can find those deleted items in the 'All Mail' folder of Gmail. That folder contains the master of every message you ever received, ever sent, and ever deleted. If you have the 'All Mail' folder visible in the Mail app, you can do a search for any message using that folder.
He goes on to explain, "Now let’s say you want to clean up the messages in 'All Mail.' First, click on that folder to select it (it will be highlighted). Next, enter a keyword or phrase in the search field (upper right corner of the Mail app) and hit the return key. In the list that is returned, select the items to be deleted and click on the 'delete' icon in the Mail toolbar. Those items will disappear from 'All Mail.'
"You’re not done. What you just deleted will now show up in the Trash folder of Mail. Empty the Trash. If you don’t do this, then when you do another search with that keyword in 'All Mail,' they will show up again, and that could be frustrating." And if this tip is handy mention it to John. He finds lots of good stuff for us.