So frustrated with mail. My email just stopped updating. The only way to get new mail to show up was to quit mail and relaunch...EVERY TIME!! I know a lot of you are having the same problem, so I am posting this to hopefully help some of you. It worked for me!!
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData/Envelope Index
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 (command-V), then press return.
A Finder window will open with the file "Envelope Index" selected. Move that file to your Desktop, leaving the window open. Other files in the window may have names that begin with "Envelope Index". Move those files, if any, to the Trash.
RESTART your computer. 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.
Test. If Mail now works as expected, you can delete the file you moved to the Desktop.