rhythmisaprancer

Interesting! I tried creating a new account with my mailfence email and it just told me no. There was a point where I experienced places balking at me using that domain, perhaps they have had to tighten up registration.

I posted about it in another recent thread. Long time user and I am happy. But I only use some features despite paying. They historically had free accounts, too, but unsure what they offer now.

A comedian had a whole bit about something like this.

I have used mailfence for 10 years or so. Not non profit (I think) but should fit your other needs.