I'm using Mojave, but I think the file structure is the same for other versions of the OS (as described below).
Apple's Mail.app saves its files in:
home/library/mail
So...
Go to home/library
And click on the Mail folder ONE time to select it.
Then do a "get info" on it (command-i).
Does it give you a folder size?
Is that folder size 7gb or so?
We want to confirm that this folder is the one "taking up the space".
If it is, and IF YOU'RE SURE you don't want this stuff any more, you could do this:
Open the "Mail" folder.
Type command-A to "select all" (everything that is INSIDE the folder)
Drag it all to the trash.
Empty the trash.
Then it should be gone.