@alyssa Not costing money is a hard one. Tuta (née Tutanota) is probably the main one that comes to mind? (Assuming you don't want Outlook.com, or any of the other classic AOL/Yahoo!/similar providers.) Proton offers a small free service (1 GB) but I don't trust its security claims over any other services, probably fine if you don't rely on them. Fastmail is by far the most popular Gmail alternative server I've seen recommended, but it's not free.
@alyssa I guess I had forgotten what Tuta claimed, I think it used to make more reasonable claims and Proton used to make more outlandish claims / implications, a brief review of both of their websites seems to show them mostly at par (though I'd still say Tuta is a *little* less aggressive than Proton). Both probably try, but given the ecosystem I wouldn't rely on either's complete privacy / end-to-end encryption as such. (I wouldn't rely on Gmail for that either but it doesn't advertise that.)