If you are already signed in, you can read your account email straight off the screen instead of guessing.
On a phone or tablet
Open the Ultimate Backup app. Tap Settings in the bottom-right corner of the app, then tap Account. The email address is the first line of the account form:
On a computer
Click the Settings gear at the top right of the Ultimate Backup app. The menu shows the address right next to the words My Account — for example My Account (name@example.com) — so you can read it without clicking anything. Clicking that line opens your account details.
What the address is for
It identifies your account when you sign in and when you contact support. It is not tied to the drive or to the files already backed up — your photos are on the drive whether or not you can remember the address.
Want to change it? See How do I change my account email in the app?
If you need any additional assistance, please contact Ultimate Backup Support.