Email campaigns are sent to users on an account, not contacts.
If you want to send email campaigns to account contacts, you'll need to add the contacts as users on the account.
For more information about contacts and users, refer to these articles:
- Users overview: /hc/en-us/articles/4406958207383
- Contacts overview: /hc/en-us/articles/4406958289559
- How do contacts differ from Business App users?: /hc/en-us/articles/4406953514647
- Add a user to an account: /hc/en-us/articles/4406951619479