When sending out an email to clients you have three choices of who you can target within an account.
In your email builder, hover over the "To:" field and click on the pencil icon that appears:
Here, you will be able to select your recipient from the three following options:
Account > Primary: The email will only be sent to the primary email address on the account.
Account > Primary and Secondary Contacts: The email will send to the primary and secondary contacts within the account.
Account > Only Secondary Contacts: The email will only send to the secondary contacts within the account.