It's also transactional emails, but Send In Blue has a free tier with 300 per day.

https://www.sendinblue.com/pricing/

I use sendgrid to send mail I host on Linode I manage with Cloudron which works with sendgrid and others out of the box. Cloudron is a cool paid server management app but their free tier includes email. Literally give the app your DNS provider details and API key, your sending service details (or internal smtp if you hate it when people actually recieve your email) and the rest is 100% automatic. The free tier limits how many one-button-press apps you can install. They're all regular open source apps, but they nicely maintain docket images for them all and the setup, including DNS for subdomains, is automatic and really smooth. I havr nothing to do with them but I was surprised by how smooth it was.

There’s more services that have free tiers but I doubt that is sustainable and I’m really not interested in periodically rushing to fix things if the next service starts turning the thumbscrews.