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

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 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.