If the problem remains, return the folder to where you got it replacing the one that is there.

As it starts up it is trying to sync 100s of emails from my account then after around 20 seconds it quits.

I have sent numerous error reports to Apple and updated to Sierra but it still wont work THANK YOU.

After reading your post, I understand that your Mail app keeps quitting unexpectedly on your MacBook.

I rely on e-mail every day, so I know how important it is for the app to be working.

Make not of the results, then restart the computer and try opening the Mail app again.

With that Finder window as the front window, either select FinderViewShow View options or go command - J.

That should make your user library folder visible in your userhome folder.

Go to LibraryContainerscom.apple.mail.

Move the folder com.apple.mail to your desktop.

If the problem is solved, recreate any required Mail settings and import any emails you want to save from the folder on the desktop.

Also clear contents ( you can use command A to select, command Delete to delete ) of folder caches, cookies, saved application state into the trash.

Please follow the article: macOS Sierra: If an app freezes or quits unexpectedly I hope so it might be help out.

Yahoo Mail App For Mac Laptop