Hello Mogi803,
Welcome to Apple Support Communities. We're happy to help with the trouble you're having with the Mail app on your Mac. To isolate this issue you're having, we'd like to have you create a new user account to test this issue. Follow the steps here to do so:
Set up users, guests, and groups on Mac
You can also use the Mail Connection Doctor to detect any issues you are having with your Mac mail. Open the Mail app, then choose Window > Connection Doctor from the menu bar. Any trouble detected will produce a red dot next to the feature that is having the issue. You will have the option to "Show Detail" of anything that is found. Let us know the outcome, and we can go from there if needed.
Good luck!