According to Zoho's web site they don't have a free option. The cheapest one is one euro/month per user - not expensive, but not free.
It's not that easy to find, but it's there. As long as you don't care about using third-party clients (free plan has IMAP/SMTP disabled), it's a viable option. I've used it for a year or two before I've switched to FastMail and it worked fine.