Ok, close Mail app. In Finder, hold down Option key, click on Go menu and select Library. In Library folder, open Preferences folder and trash com.apple.mail.plist, com.apple.mail.plist.lockfile if you see it.
Then go back to Library, open the Containers > com.apple.mail > Data > Library folder. This time open the Saved Application State folder and trash the folder/file "com.apple.mail.savedState".
Open Mail. Is it still freezing ? If so, then go back to Library folder and this time, trash Mail folder.
Open Containers folder and trash com.apple.mail
Open Caches folder and trash com.apple.mail if you see it there.
Open Mail and let me know how it is going.
Note : Do not empty Trash