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

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.

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.