Apple mail has become very buggy

Hi sdb2013,

Thank you for participating in the Apple Support Communities. We understand the Mail app isn't working properly. You won't be able to delete the Mail app as it's part of macOS. However, we recommend updating macOS as this will update the Mail app and may help to resolve some of the issues you're having. Here's how to backup and update your Mac if needed: Update macOS on Mac.

"Before following these steps, it's a good idea to back up your Mac.

1. From the Apple menu  in the corner of your screen, choose System Settings or System Preferences.

• If you chose System Settings, click General on the left side of the window. Then click Software Update on the right.
• If you chose System Preferences, click Software Update in the window.

2. Software Update then checks for new software. 

• If Software Update finds new software, click the button to install it. The button might be named Update Now, Upgrade Now, Install Now, or Restart Now, for example. You might then be asked to enter your administrator password.
• If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only software that is compatible with your Mac."

If you notice this is happening with a specific mail account, you may want to delete the account and add it back again to see if the issues persist after doing so. This article has the steps: Log out of or remove email accounts in Mail on Mac.

"You can remove an email account from the Mail app on your Mac at any time. When you do, the account’s email messages are removed from your Mac. Copies of messages remain on the account’s mail server and are still available (from webmail, for example). 

Important: If you’re unsure whether messages you want to keep are still on the mail server, move or copy them to a mailbox stored on your Mac (the mailbox appears in the On My Mac section in the Mail sidebar) before you remove the account in Mail.

1. In the Mail app  on your Mac, choose Mail > Settings, then click Accounts. 
2. Select an account, then click the Remove button .
3. Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts settings. Click the button to open Internet Accounts, then turn Mail off for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button ."

We hope this helps.

Take care.