Zoho Mail will do up to 10 users for free on one domain. They're my (less favorable) go-to after Google and Outlook dropped their free custom domain support.
Thanks will check them out.
Might not apply, but if you have a free legacy account at Google, you can host multiple domains on that account.
I also use Zoho. I create my email addresses, configure them to forward emails to an @gmail account and then configure gmail to send emails from my @customdomain.com account. I get free custom domain emails and I get to use gmail for email management. Zoho is basically just the middleman.
Try Banckle Email. They offer a free plan with custom domain support, IMAP/POP3 and web client - plus APIs if you fancy using their service in your app.
I've been using Google for years. Love it. (with several domains)