mail app not working

Are there any other Mail accounts there?

If not Quit Mail & delete this file...

~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

~ tilde indicates hidden Home Library.

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Method 2:

1. Go to the Finder (or desktop).
2. Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
3. With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.

 4 Select Library from the Go menu (while holding down Option) to access the hidden folder.

Log into your WebMail in a Browser using the same info, does that work?

Is it an IMAP account or POP?

In Mail>Preferences>Accounts, inder the name is the account type.

Was it ever setup on this Mac?

In Sys Prefs>Internet accounts is it there to remove first?