After configuring your Zoho Mail Add-on as a shared account, you need to enable sharing option for each Contact or Lead.
Even while configuring your Zoho Mail as private account, you can view mails of other users provided they have enabled
Any change in your Zoho password should be updated in the Zoho Mail configuration page, else you will not see the emails in Leads/Contacts modules.
Leads and Contacts with no email address will not display any mails in their email section.
Changes made in the email address (for Leads or Contacts) will fetch only the mails linked to the new email address. Mails linked to the previous email address will no more be available in the CRM.
Change in the Mailbox (email address configured for Zoho CRM) will not display the emails of the previously configured Mailbox. These emails will however be available in Zoho mail.
Emails for the deactivated users will not get displayed in the Leads or Contacts page. However, the emails can be viewed in the individual's mail account.