You can designate one or more text fields from your Contact Forms to be Email Address fields that are used for Emailing Contacts.

Makerble will use the email addresses that are designated as Email Address fields whenever you:

Note: Sending Unique Survey Links

You don’t need to designate a field as an Email Address field in order to send unique survey links to contacts or observers.
Makerble will automatically detect the first email address that is saved for each contact and send the survey link to that email address.

Steps

Only an Organisation Admin can designate text fields to be Email Address fields

Select My Apps in the top menu
Select Control Panel
Scroll down to Account section and select Preferences
The Activate Email Messaging tick box should already be ticked. If it is not ticked, do tick it.
In the next box, select the Contact Form fields you would like to designate as being the ones that hold the email address of your contacts. You can select multiple fields
Press Save.
Any contacts who have an email address in one of your designated fields, will now be available to receive emails when you are on the Create New Broadcast page

Makerble User Guide