Hi TabMe,
For help with opening messages with the Mail app, test your connection using Connection Doctor. The steps below found in the article here can assist: Use Connection Doctor in Mail on Mac - Apple Support
"Use the Mail Connection Doctor to check your internet connection or the status of your email accounts.
Tip: When an account or network is offline, a lightning bolt icon
or a warning icon
is shown at the end of the Favorites bar (just below the search field) or in the Mail sidebar. You can click the icon to try and take an account online, or to view status information.
- In the Mail app
on your Mac, choose Window > Connection Doctor.
2. Review the status information.
If you’re connected to the internet, there may be a problem with the account’s mail servers or your account setup. Follow the instructions shown in the Details column. To quickly display the account in Accounts preferences in Mail, double-click the text in the Details column.
To show log information while Mail tries to connect to mail servers, click Show Detail, then click Check Again.
3. When you’re done, click Check Again to verify if your actions resolved the problem.
If the problem persists, contact your email account provider to verify your account details and setup."
You can also test opening messages while booted to safe mode. The article here has the steps to assist: How to use safe mode on your Mac - Apple Support
If you continue to have issue or can't further isolate after testing in safe mode, reach out to Apple Support directly to further assist.
Contact - Official Apple Support
Contact Apple for support and service - Apple Support
Best.