I've been a long time user of Gmail and carefully watched Zoho products evolve.
I've tried once or twice to make the switch from Gmail to Zoho, but there's always been one or two small things preventing me from doing so. Therefore, I've decided to help and let you know what bugs me and what I think culd be improved. It may be of use... one nevers knows.
I'l start off by saying that the UI is a win for me. It's great having the possibility of a preview pane and multiples tabs for emails. This way i can have various emails open at once without having to open and close emails to compare, edit etc.
I do however find fault with composing an email. I have contacts with various emails, but for some reason, when I start typing the contact name, only one email appears. It should be more like gmail that shows the various possible emails for that contact. It makes no sense whatsoever having to open contact pane, search for said contact and click on desired email just to send a quick mail.
While on the subject of contacts, search could improve if implementing a filter with autocomplete. While typing in the search field a name, it would be great to have the list narrow down (filter) matching the search text, rather than wait for us to hit "search".
Folders + Labels, great!!!!. Possibility to access multiple pop account, great!!! Each pop account has separate "folder"/"account pane" excellent!!! Not being able to move emails from one account to another, not great!! A relative show stopper for me. It would be great to be able to move emails from one account to another. Even better would be the ability to choose whether an account gets a separate pane or gets imported into a global account pane.
Search could also see some improvement. Rather than mimick a desktop app's search (thunderbird jumps to mind when I see Zoho email search), leverage the power of the web. The use of search operators makes more sense and is quicker. Search betweenm before, after date is very important too..
Apart from that, you have stellar products and I surely hope your outstanding work keeps getting better.
Cheers,
A hopeful Zoho convert.