Q: El capitan issues with Mail

Hello Annito,

Thanks for that info and choosing the Apple Support Communities. I know how important getting Mail to work properly again is to you! If I understand correctly, you are unable to finish opening Mail as it says upgrading. Since the steps you have tried didn’t resolve your issue, I would use these steps to get Mail working again:

1. First, try closing Mail and re-open while holding the Shift key on your keyboard, as this temporarily prevents an app from re-opening its documents and windows.

Automatically re-open windows, apps, and documents on your Mac

2. Use Safe Mode to start up your Mac so that it performs certain checks, delete system cache files, verifies your startup disk, and attempts to repair directory issues if needed. Then test Mail in Safe Mode, and if the issue is resolved, restart normally and test Mail again.

Try safe mode if your Mac doesn't finish starting up
Limited graphics performance in OS X recovery or safe mode

3. You can figure out if unexpected behavior is related to user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and testing for the issue.

How to test an issue in another user account on your Mac - Apple Support

Cheers!