Greetings snshimota,
Welcome to Apple Support Communities. Per the Apple Support article: If Mail on your Mac can’t connect to your email account - Apple Support
If you entered the correct password for your email account in the password dialog in the Mail app
on your Mac, but Mail indicates it can’t connect to your email account, your email account provider may be rejecting the password.
You can also try this step to help you with this issue:
Check your password in Internet Accounts preferences
Depending on the type of email account you're using, you might be able to correct the password as follows:
1 . Quit Mail. If it doesn't quit, you can force it to quit.
2. Choose Apple menu > System Settings (or System Preferences), then click Internet Accounts.
3. Click your mail account.
4. Click the Details button, if present.
5. If you see a password field for your account, delete the password and type the correct password.
6. Close the settings window, then open Mail and try using your account again.
If Mail still asks for your password, the password is probably incorrect. Your email provider can help you verify your email settings and make sure that you're using the correct password.
You can find more tips in the Apple Support article:
If Mail on your Mac keeps asking for your password - Apple Support
We hope this helps you.
Feel free to reach back out if you have any questions.