It's good to know you use it yourself :) I use, because of one of my biggest clients, your mail client every day as well.
Already presented my disclaimer: I have been working with Gmail since it's inception. Before that I used all kinds of different clients; all not very nice. The best I used was mutt by far. But that pre-gmail ;) I'm talking about the client, not about Google as cloud mail storage provider, although I really don't think they are evil yet as people seem to think.
I digress.
So this is more a shootout than a face value comparison, although most points, I believe, would annoy me as well without the gmail comparison.
Biggest: speed. Basically that's my biggest issue. ZOHO is slow. Very slow. This is not only the mail app, this is everything (the project management app makes me bang my head against the wall every week I have to enter hours as it takes... yep, hours...). But for the mail app, something I use intensively, this is really quite unworkable. My colleagues at the client all use Outlook with Zoho as they cannot work with the mail client as it is too slow to really work with.
With gmail you click on a message and as by magic, instantly it opens the entire thread. For the current page even when your network is gone. With Zoho you see 'loading' then you wait. And wait. And then it appears. Next mail. Loading... The agony. It's like it was the year 2000 and we just had Ajax.
No internet detection; when internet is down even for a little bit, Zoho mail becomes unusable. It takes forever to notice internet is down and it will be showing 'loading' forever and nothing works even if you are back again. Refresh fixes it, but it seems brittle. Sometimes the CSS breaks, sometimes it just give random error messages when you click on anything. Even though the internet was back for a while already.
This is probably a matter of taste, but screen real estate; when i'm reading a message i'm not interested in the rest of my messages. I want to use most of my available screen for reading that message (like gmail...).
Spam: I get tons of spam; I have ancient email addresses and they attract 100s of 1000s of spam mails per month; in gmail I notice NOTHING of this, really absolutely nothing. In ZOHO the spam filter is almost not noticeable for me, I keep clicking spam until my wrists are locked up.
And then the little things, the ergonomics; it just feels clunky. Not as bad as the project management app (I am no interface designer, but come on :(, but it just feels old and heavy. If Google taught anything for mail client designers; not everything has to look/work like Outlook.
This all makes me say ZOHO web mail is not ready for heavy mail users even though a lot people use it for that; a lot people also use Lotus Notes, that doesn't prove much. I will pay more attention and mail you with my findings because now I only see the big points clearly, but the whole experience is actually just 'not good'.
You did a marvelous job of building this company; my compliments. You have a lot of good products and you managed to build this impressive array of products at an amazing rate. But that has it side effects; polishedness is one of them. I know a lot of people who work with Zoho products every day and for some of them they still cannot find how to do some things ( I won't name my nemesis again here ). Would it hurt to add employee 1401 who is a $400k/year UX guru to overhaul everything? I think it would make all the difference.
Thanks for replying here!