A couple of years ago my parents wanted to trial individual email for their very small business (just a few employees). Google had recently removed the free tier from Google Apps, and Zoho was the only company we could find that offered a similar service. I set it up and they used it for ~6 months with quite a few issues in that time, whether service outages or (more commonly) emails not being delivered due to Zoho's spam filter or the recipient's spam filter giving false positives. Customer support was near enough to useless.
So when we decided that it was worth upgrading to a paid plan we didn't stay with Zoho, due to the poor experience. If Zoho charged just a dollar or two for accounts it may help reduce all the problems they have with spammers abusing the service and the flow-on effects that has on their customer support.