I want to thank everyone who replied. I did some research on most of these and think PurelyMail is the winner for me. Feel free to correct me if I got some details wrong. I want to give a shoutout to @mbirth for mentioning Disroot, which looks like a really interesting experiment in federated services.

Also, I know this post is really bending the rules for c/selfhosted, but connecting your selfhosted services to an email provider is essential, and having a reliable and affordable email provider just makes this weird hobby of ours a little easier.