Open Mail. Select Preferences from the Mail Menu. Click on the Accounts button in the toollbar. You should see your mail account listed in the sidebar of the display. Click on it to select it then click on the Delete [-] button at the bottom of the list. Navigate in the Finder to the /Home/Library/ folder and look for the Mail folder in the window. Select the folder, CTRL- or RIGHT-click on the selection, and select Move to Trash from the context menu. Next navigate to the Preferences folder. Double-click on the folder to open a Finder window. Locate the entries named com.apple.mail....plist. Select them and drag them to the Trash.
This should reset Mail.
Four Ways to Make the Home/Library Folder Visible
A. This method will make the folder visible permanently. Open the Terminal application in the Utilities' folder and paste the following red colored command line at the command prompt: chflags nohidden ~/Library then press RETURN.
B. Click on the Desktop, press the Option (⌥) key, select Library from the Finder's Go menu.
C. Select Go To Folder from the Finder's Go menu. Paste the following in the path field: ~/Library and click on the Go button.
D. In Sierra activate the Finder, press COMMAND-J, set Finder display to Icon view if it is in some other view mode, select your Home folder from the sidebar, check the box labeled, "Show Library Folder.