You should use PGP so your messages can't be read.
When signing up for services I try to use a new email for every service so that one email can't be used to track me.
Besides that the ones I use:
cock.li, elude.in, protonmail.com, danwin1210.me, and a few others. These all work when registering through Tor.
Why free though? Renting a domain can be done for cheap (~20USD/y); and email management is a real service with its own set of challenges. Why would anyone do that 100% free?
Trying to give fixed income seniors trusted free options to create additional email addresses. They are freaked out about Gmail. Not sure what it would take to create such a service.
That should be good to start you off.