Galaxy S7 - How to back-up stored email mailboxes with ADB?
What you want to do is simply impossible. The app Samsung Email (com.samsung.android.email.provider) contains the entry android:allowBackup="false" in it's AndroidManifest.xml.

This entry totally disables adb backup for all of it's app data. What ever you do the adb backup will therefore never contain the email data.

Of the app has no integrated bulk backup/export function for the whole mailbox you can not gain access to it's app data unless you have a rooted device or you use a root exploit.