You can use the 'Flag' feature in Zoho Mail, to add flags to the emails. The types of flags available are:
You can add a flag to an email, directly from the listing pane by selecting the flag icon in the email.
Select the multiple emails that you want to add flag to.
From the menu, select 'Mark As' and choose the flag you want to apply for all the emails.
You can click the flag icon on the email, to remove/ clear the flag added for the email.
You can view all the emails for which you added from the Views >> Flagged option. All the emails that have some flag, across the folders get listed in this view. The view also provides you the details of the folder, like the folder in the email is placed etc for easier access.
You can also view the flagged emails in a folder, by choosing 'View All' >> Flagged option from the specific folder. You can choose a particular flag, to further filter the emails listed.