Email Keeps Closing

Open  (Apple menu) > System Preferences... > Internet Accounts and remove your email account. Then, add that email account again. Those steps are described in Add or remove email accounts in Mail on Mac - Apple Support. Follow them in reverse order—in other words remove, and then add that same account.

If you have more than one email service provider, remove them one at a time. Launch Mail after each account removal, to determine if you removed an affected account.

After you isolate the affected account, use its webmail interface to find and delete the message(s) causing Mail to crash. That may take some time depending on the number of email messages you choose to keep.

Once the problematic messages are deleted, only then should you add that account to Mail again. That action causes its Inbox to synchronize with that email service.