In addition to the Email address field, a Secondary Email Address has been provided for leads and contacts.
You may be using Mail Add-on to fetch your business emails and associate them to the leads and contacts in Zoho CRM. With this additional secondary email address, all the emails sent/received to/from the secondary email address will also be listed in the Emails Related List of a lead/contact.
Scenario: The lead/contact has changed his/her email address. You want to update this new email address in your Zoho CRM.
Problem: If you change the value in the email field with the new email address, all the mails associated to that email id will no longer be listed under the respective records.
Solution: With the secondary email address, you need not replace the lead/contact's email address. You can just add the new email address in the Secondary Email field. By doing so, the mails associated to both the new and old email addresses will be listed under the respective records.