When sending out an email to clients - you have 3 choices of who you can target within an account.
If you are in the email builder and click on the pencil icon at the end of the To: line it will give you three options:
The first option is Account: which means the primary email address on the account.
The second option is All Contacts: this will send to ONLY the contacts within the account.
The final option allows you to send to ALL emails within an account - both the primary and all the contacts.