Yeah, Apple has bass akwards way of choosing your category. :(
Not that I think deleting that file should be your first step, but the reason you can't find it is that Apple hid the Users' Library folders...
~ tilde indicates your user library folder
Method 1:
- From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
- In the window that opens, enter ~/Library, and click Go.
Method 2:
- Go to the Finder (or desktop).
- Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
- With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.
4 Select Library from the Go menu (while holding down Option) to access the hidden folder.
I'd do this 1st...
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.
If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.