Mail box problems

Question

Every y=time I tell it to get new mail it seeks to download all 5000  messages on the server again.  This is pop mail from ATT server .   I have deleted years of old mail to get it down to 5000 messages but the mailbox keeps trying to send all 5000  each time.  In other words  if  I quit mail and come back .  and say get new mail it will seek to deliver all 5000   if that times out and not all coe down  and then I tell it to again get new mail it starts over to send 5000 ??   Help. 
I thought about deleting the mailbox but that is greyed out even though I am an admin on this box ??

Answer 1

Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData/MessageUidsAlreadyDownloaded3
Right-click or control-click the line and select
          Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Quit the application if it's running. Move the selected item to the Trash. Relaunch the application and test.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select
          Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.