I am a heavy user of labels. I use them to filter mails from my different customers. When I receive an email, I automatically label it with the company name of the contact it is coming from. But when I reply to a message, the reply is not labelled automatically.
Zoho Mail does not give you the opportunity to label a message when you are writing it, or when you are responding to a message. You can only label messages that have been sent or received. Labelling all of the messages in my sent box is very time consuming, so I typically don't do it. This diminishes the value of the labeling feature, since when I use it, it only brings back messages that where labelled when I received them.
I would love to have available in Zoho Mail the possibility of enabling a "Auto-Label" feature, which basically tagged all email responses with the same labels as the original message. I would also like to be able to define the label of a new message at the time I am writing this message. These enhancements would save me a huge amount of time, and make the labelling feature much more useful.