Mail keeps showing a spinning beach ball

Try booting to Safe Mode. That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

A lot of times just boot to Safe Mode followed by normal boot resolves problems.

If no problem in Safe Mode but problem returns after a normal boot that mean something is being loaded during a normal boot that is causing the problem.

There may be an issue with one or more of your email accounts.

To resolve this, give the following steps a try:

1. Temporarily disconnect your Mac from the network. (Note: You don't want the Mail app to try to download messages for each of your accounts at this point.)
2. Launch the Mail app, and then, go to Mail > Preferences > Accounts > Account Information.
3. For each of your email accounts, disable (uncheck) the Enable this account option.
4. Close the Accounts pane, and then, close the Mail app.
5. Relaunch the Mail app, and then, reconnect your Mac to the network.
6. Return to Mail > Preferences > Accounts > Account Information, and then, re-enable one of your email accounts. Allow it to download any messages. It should be completed doing so when you see the Checking for Mail... disappear in the lower-left corner of the Mail app's window.
7. If successful, re-enable the next account and continue doing so one-at-a-time if successful. Stop at the account where the issue reappears. That would be the account that will need further troubleshooting.

It is impossible to follow the instructions in #2 ABOVE, WHEN GOING TO PREFERENCES I GET THE SPINNING BEACH BALL SO CANNOT FOLLOW THE INSTRUCTIONS DUE TO THE PROBLEM.