Go for Yandex 360 (formerly Yandex Mail for domains)
Key points:
- Absolutely free
- Add your domains
- Set DMARC, DKIM & SPF
- Almost unlimited storage for emails
- Adding multiple users (email accounts) free
- Additional features included like Drive, Docs etc.
Note: We use it for FreeTools.Dev (https://freetools.dev) and it works very well (such as SMTP delivery)