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.
Yeah this was my thought also, although i understand the backup option for recovery, but thats already present with a phone verification.
I actually do have other email addresses, i'm just surprised by the irony of it, especially that there is no workaround, for a tech company powerful like microsoft, making just viewing personal email via their service so unpleasant..
It's a bad system all the way around. I've been debating a few moves and I've been kicking the can down the road, because I'm worried about some of these sharp edges.