In order to bulk email a segregated client list, you will need to first make sure that you have Mailchimp integration. This integration can work with Contact types, which can be added or removed from select contacts at any time.
You will then need to add/create a new contact type for the contacts that you would like to bulk email.
To do this, in the Web Portal go to Contacts > Select the Contact > Edit > Select the contact type, or type a custom contact type in the box (this will create/save the type for you to add to other contacts (see images below).
To sign up to and learn more about Mailchimp, go to: www.mailchimp.com