Had the same problem, and it’s definitely not the first time I’ve encountered this issue. I have an inbox attached to a custom domain, and there have been multiple instances of platforms refusing registration, as they think it’s a temporary inbox, or just plain give you “this isnt gmail or alternative”.

If reducing fake accounts is the goal, this ain’t it

I guess you can talk to Steam Support about it but there might be some other reason for this.

I remember having tried to use a service before, and after trying my email address I got the error message that only hotmail, gmail, and icloud email addresses were allowed. Luckily I didn’t really need the service, so I noped out if there real quick.

Have you tried using the steam mobile app? I think that has some advanced recovery options. Unsure if you can change the registered email that way, but might be worth a shot.

Edit: Specifically the Steam Guard feature.