- Contacts: individual email addresses
- Properties: custom properties for your Contacts
- Segments: groups of Contacts for your organization
- Topics: user-facing tools for managing email preferences

Contacts
Contacts in Resend are global entities linked to a specific email address.
- is associated with a single email address
- can have custom properties
- can be in zero, one or multiple Segments
- can be opted in or out of Topics
upsert or skip existing Contacts.
Learn more about Contacts.
Properties
Contact Properties can be used to store additional information about your Contacts and then personalize your Broadcasts.
first_name: The first name of the contact.last_name: The last name of the contact.unsubscribed: Whether the contact is unsubscribed from all Broadcasts.email: The email address of the contact.
Segments
Segments are groups of Contacts for your organization. You can use Segments to send emails to a specific group of Contacts using Broadcasts.
Topics
Topics are user-facing tools for managing email preferences. You can use them to manage your Contacts’ email preferences.