I am one of their customers. Often buggy (webmail) and issues with availability. Suffered a prolonged DDoS attack last year which left me a whole weekend without email. Upgrades seem to often cause outages. Some issues in Operational security. Also it is unclear if data at rest is encrypted. They use SSO with all their other apps (makes sense) but then support asks you to give them your password so they can verify any issues. (and they don't comprehend and treat you like an idiot if you say this isn't possible). Support issues generally take ages to solve and you have to mention the CEO in your tweets to get any response. Also even you have a premium subscription there is a limit on the number of mails you can send per day from your account. This might be an issue for companies with marketing or newsletter accounts and wish to build campaigns for this.

I'm seriously questioning their technical competence not just from support but also from development. When I subscribed to Zoho first thing I noticed (in 2011) that sending emails with multi-part alternative MIME types results in an empty plain-text part so anyone reading the mail in a plain-text MUA would see just an empty email (and it took them ages to get this basic feature fixed).

here some of my recent arguments with them: https://twitter.com/search?q=%40valbonneconsult%20zoho&src=t...

On the upside they are pretty easy to set up with IMAP and TLS in more exotic environments (e.g. offlineimap, postfix, mutt etc). If you go with them don't expect to scale your business because it's going to end in tears. OK for private use IMO.

I still use Zoho until now because of integration with some other apps (once I migrate those I will turn my back on them and never look back).