Zoho Mail worked well as a provider for my email on my custom domain. It has all of the features you would need out of a typical email provider, like Gmail or Outlook. It was simple to set up, just by adding an MX record into my DNS which took until 5 minutes. The UI is simple but powerful, and their advanced search tools helped me find emails from long ago that I wouldn't be able to find otherwise.
Since my only user was myself and an additional service account for miscellaneous sending purposes, I was using the "free" version, which is fully featured, but limited to 5 users. While the free version includes quite a lot of mail features, it is ultimately limited on mail storage, eventually causing me to move away. The free version also included access to some of Zoho's other products, including their contacts and calendar suite (expected with any email provider these days), as well as their office suite (think Word/Excel/Powerpoint). These were nice features to have included, but I ultimately did not use them too much, except for Contacts and Calendar.
Zoho is the underdog in the category of SaaS business suites, competing against GSuite and O365. While the product is fully featured and includes plenty of powerful tools, what it lacks is widespread support and integration, which ultimately made me move to O365, after using Zoho for 5 years. Their propriety storage and document solution are basically unheard of compared to Office Online (with OneDrive) and Google Docs (with Drive). Their IMAP/POP3, CalDav, and CardDav solutions are basic and fragmented, though the features are there. Furthermore, with the free account, I didn't get/expect any customer support so any problems were left to myself to fix (not many came up anyways).
Primarily, you lack integration with third parties and industry leaders with their services. They don't provide many plugins or connectors to external parties, and when they do, the options are lackluster. Don't expect to be able to automate things with Zoho.