I have run into far too many email services that require an email service to set up email. It always made me wonder how someone is supposed to get their first address... or how someone can live with one address, which should be a perfectly normal thing.

I signed up for Fastmail today and to my pleasant surprise, they didn't ask me for another email address. Though they did ask for my phone number as an anti-abuse auth.

That said, Fastmail is paid, so if you need a dummy email just to check a box that you have one in Outlook, something like Proton, Yahoo, or Gmail is probably better... assuming they let you get through the setup without providing another email address.

If you happen to have an Apple account, you can enable iCloud email on that as a secondary address as well.