If you see a file called "inbox" inside your "mail" folder, use this method to delete default mail buildup.
1) Login to your control panel.
2) In the Files section, click File Manager .
3) In the popup select "Web Root" and "Show Hidden Files"
4) On the left, click on the word "mail" next to the mail folder (all of the files in that folder will appear in the right panel)
5) Click on the word "inbox" in the right window to select it.
6) Click "Delete" on the top navigation bar.