Hello CA_Dreamer,
We'd be happy to help. There are a few troubleshooting steps we can try. To get started, we recommend using Connection Doctor to help identify any potential issues with your accounts. Use Connection Doctor in Mail on Mac - Apple Support
Connection Doctor may not show any issue if your accounts are able to send and receive. If you do see an issue, follow the steps it recommends.
You can also check and if need be change your account server settings. iCloud mail should automatically add your server settings, but it's not a bad idea to check for duplicates as that may cause this issue. Change Server Settings in Mail on Mac - Apple Support
As a last resort, you can try to sign out and sign back in. This will remove the accounts from your Mac and set them back up. This will also remove any email you've saved offline. Log out of or remove email accounts in Mail on Mac - Apple Support
You may find these steps helpful:
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 .
Thanks.
