Here’s how to fix the spinning beach ball (wheel) issue in Mac Mail, it involves resetting the Mac Mail Preferences by trashing the file.  This has been tested for Mavericks and the file may reside in a different location based on the version of OSX you are using.  Do so at your own risk.

Step 1 – Open Finder

Make sure Mac Mail is not running and force quit if necessary.  Open a new finder window.

Step 2 – Delete the file

Go to /Library/Containers/ and find the file.  Click and drag the file in to your trash.

Step 3 – Restart your Mac

This one’s important – Restart your Mac and open up Mac Mail.  In most cases this will reset your Mac Mail to it’s default view and it should be good to go.  Some people have reported that they have had to add their email accounts in again so be prepared for this particularly if you don’t know how to set them up by yourself.