If Mail on your Mac keeps trying to upgrade your email library

Follow these steps if Mail in OS X El Capitan doesn't complete the “Upgrading Library” process. 

When Mail in El Capitan upgrades your email library, it displays a Mail Upgrade window with an “Upgrading Library” progress bar. If Mail can't complete the upgrade, it might quit, reopen, and attempt to upgrade your library again. Follow these steps if that upgrade cycle continues repeatedly:

  1. Force Mail to quit, such as by pressing Command-Option-Esc.
  2. From the menu bar in the Finder, choose Go > Home. Your home folder opens.
  3. Choose File > Get Info. An Info window for your home folder opens.
  4. If the “Sharing & Permissions” section at the bottom of the window isn't open, click the triangle in that section to open it.
  5. If the Lock button in the lower-right corner of the window shows a closed lock  , click the button, then enter your administrator name and password. 
  6. Click the Action menu   in the lower-left corner of the window, then choose “Apply to enclosed items.” Click OK to confirm the action.
  7. When the progress bar at the top of the Info window closes, open the Terminal app. It's in the Utilities folder of your Applications folder.
  8. Paste or type this command in Terminal, then press Return:

    diskutil resetUserPermissions / `id -u`
    On U.S. keyboards, the ` character is just above the Tab key.

  9. When the process completes, quit Terminal. You should now be able to open Mail.
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