I am not sure what other versions of mail have this problem. I am using OSX El Capitan, and Mail version 9.1. I still needed to delete the files from the hard disk manually.

Go into, then go to Settings on the top right menu, then under the ‘Labels’ tab I changed ‘All Mail’ to ‘Hide’ Here’s how to disable the ‘All mail’ folder from being downloaded to your Macintosh. No wonder my internet usage was high last month – my laptop must have downloaded the entire 15GB of emails from gmail. The ‘All mail’ folder had over 17,000 emails in it. Yes Apple mail now by default seems to download the gmail ‘All Mail’ folder which included every email you’ve ever deleted on gmail. I discovered 15GB of gmail data in Apple Mail. This huge multicoloured box turns out to be the Gmail ‘All Mail’ I could do this pretty confidently because my macbook air is my second machine, all my photo’s are only home computer if anything goes wrong. While I was at it I went to my ‘Applications’ folder and deleted the old ‘iphoto’ app which took up 1.7GB. I just dragged this to the trash and saved 6.6GB. To find the file just right click on the box and select ‘reveal in finder’: Deleting my old iPhoto library released almost 7 G of space! El Capitan came with a new app called ‘Photos.’ The new Photos app imported my old iPhoto library, but it didn’t delete it! They are the same photos, the entire library has been duplicated. The large turquoise section to the left is the new ‘Photos’ library. Look at this screenshot from Disk Inventory X: The pink section surrounded by the yellow line is my old ‘iphoto’ library. I ran Disc Inventory X to see where the excess space was taken. The reason why the storage optimization feature excels in the macOS Catalina is that the device has an enhanced space for additional storage. To run well a mac hard disk needs at least 20% free. Once activated, the optimizer will automatically arrange your files, removing any program junk to free up space.

I just noticed after El Capitan that I lost a lot of space. My MacBook Air is running a 120GB SSD drive so I don’t want my hard drive full of excess things I don’t need.

Osx clean up disk space