Autofill contacts:

In email composer, when you type in the recipients in To/ Cc or Bcc fields, the matching contacts be listed and you can choose the relevant contacts from the list. The email addresses of other members and groups of your organization will also be automatically listed for autofill. In case you have a CRM account, you can also directly lookup CRM Contacts and choose the recipients for the email.

@mention contacts:

When you compose an email, directly @mention the users in the content of the email, to automatically add them to the recipient list. You can either compose a new email or you can @mention the users in the replies or forwards to automatically add them to the list of recipients.

Address Book:

You can also click on the Address book icon in the To/ Cc/ Bcc boxes to view the Address book dialog. You can choose the the email addresses, which are alphabetically sorted in the Address Book. The address book has a separate section called Categories, from which you can insert email address based on Categories. 

Organization Contacts: 

The administrator of the organization has access to the Organization Contacts. When each user/ group is added to the organization, their email addresses and all the aliases get added automatically to the organization contacts. The administrator can add/ make changes or remove details from the organization contacts by logging in to https://contacts.zoho.com. 

Other users of the organization can view the Organization contacts.