My mail now says I have thousands (24,931!) of messages in my inbox

Question

I let my inbox creep up towards ~2,000 over 8 years (I know not good) then one day the system went crazy and now says there are 24, 931?? I am not sure how many are actually there! Also, possibly related, when I send a single email, the system says something in the order of sending 30 of 30 outgoing messages! My computer has become very slow!
Late 2012 iMac
running OS X 10.9.5
processor: 2.7 GHz Intel Core i5
memory: 8 GB 1600 MHz DDR3
Please help!

Answer 1

Quit Mail. Force quit if necessary.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData
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. Paste into the box that opens by pressing command-V, then press return.
A folder window will open. Inside it there should be files with names as follows:
          Envelope Index
          ExternalUpdates.storedata
Move those files to the Desktop, leaving the window open. Other files in the folder may have longer names that begin as above. Move those files, if any, to the Trash.
Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK. Typically, the process takes a few minutes, but it may take hours if you have gigantic mailboxes. In that case, you may be able to speed things up by temporarily adding your home folder to the Privacy list in the Spotlight preference pane. Remove it when Mail has finished importing.
Test. If Mail now works as expected, you can delete the files you moved to the Desktop. Otherwise, post your results.