I think we will find that majority of companies that do potentially block it would be companies we don't want to do anything with to begin with. My thinking is a lot of companies only want a single user to register once so they can track that user anyway they can.
For example, I opened up Agoda once in firefox private tab, and searched for 2 specific hotels to get an idea of pricing. As I had signed into Agoda, less than 5 minutes after searching both those hotels were listed on facebook with discounts. So with ad's and social media blocked, the only way they could link me was via email.
If everyone starts using fastmail to hide their email, then companies cannot do this targeted advertising and will block it.
FastMail does support custom domains for disposable email addresses.
They also have a pretty big library of email domains to choose from that I can create normal alias for, so I'd be surprised to not see those come as an option in the future.
For example, I opened up Agoda once in firefox private tab, and searched for 2 specific hotels to get an idea of pricing. As I had signed into Agoda, less than 5 minutes after searching both those hotels were listed on facebook with discounts. So with ad's and social media blocked, the only way they could link me was via email.
If everyone starts using fastmail to hide their email, then companies cannot do this targeted advertising and will block it.