Any useful disposable email service will only allow *receiving* emails. As a user this protects me against possible bad actors that for some reason need me to give out my email to them.
Any service allowing *sending out* mail from their mailservers will quickly find their mailservers blocked - this is also list based. A common one being spamhaus.org . The way firefox relay solves this is by only allowing to *reply* to mails you have received, and I assume they have some rate-limiting in place as well.
This is really just about making life harder, and forcing people into using an emailaddress as their identity token, which seems all sort of wrong.
Next someone will block mailservices that allow + adressing or other kinds of alias systems ?