Earlier quoted context omitted.
Fastmail's domain feature is the killer app for me. I have my @firstandlast.com secured for $7/year or something, and I can make anything@firstandlast.com and if someone starts spamming me I can just block it.
One thing I'd recommend is making throwaway email addresses using one the domains they own (I think have around a 100). That way, if a spammer gets its, they won't know your domain, and try spamming other username at the domain.
Spammers are going to find your domain name and spam it at obvious usernames anyway (eg contact@).
Besides what use-case do people have for throwaway addresses? In my experience in most cases the addresses you use aren't throwaway at all.
Fastmail does subdomain aliasing and I've been using that for years with my own domain without issues. Every subscription I have has its own email address. I don't need someone else's domains for that.