If you have never opened Mail before, you will see the screen below. To use Apple Mail, you must first check that IMAP is turned on. To verify which version of Apple Mail you have, open Apple Mail and then select About Mail from the Mail menu. If you are using Apple Mail v6, v7 or v8, please follow that version-specific guide. These screenshots are for Mail v9 (El Capitan- OS X 10.11) and may differ somewhat if you use Mail v10 (macOS Sierra- v10.12).
You can configure Apple Mail for viewing your U-Mail account by following the steps in this configuration guide.
