If you happen to be paying for iCloud storage, you could use their hide-my-email service for those rare accounts and have it send to your proton email (apple lets you use an external email to forward to). That’s one of the big benefits of apple’s alias solution; they are all in the icloud.com domain and no website would ever block that.

Wow almost 9 years old, but I hadn’t seen this before so thanks for posting.

I personally haven’t used it yet but I believe you can do all this with Matrix (choose your client - probably Element).