Changing Steam Email to My New Startmail One - Not Accepted

Hey everyone. I recently created and started transitioning to my Startmail email address. I'm changing emails for all my accounts bit by bit, as new emails get forwarded to my new inbox from my old gmail (which I plan on leaving as is with forwarding for about a year). Just now I decided to update my steam email. I first created an alias from startmail's aliases UI, but steam didnt like that. It specifically gave me this message:

It appears you've entered a disposable email address, or are using an email provider that cannot be used on Steam. Please provide a different email address.

So then I decided to add my regular startmail address, but I got the exact same message.

How can I deal with this? Ideally I would love to contact them and explain and request that they allow startmail (I mean, its a paid service so unlikely people will be generating multiple email addresses - as for aliases, they could just disallow the @use.startmail.com addresses, which are the aliases).

What do you guys think is my best bet for this issue?