The bottom of each client profile has a radio button to turn ON if clients choose NOT to be emailed marketing newsletters/emails.
By default this button will be turned OFF. If someone opts out then you would turn it ON.
From your Profiles page you can export a list of all of your clients which are 'opted in'
Select OFF from the Opt out of marketing dropdown
Check the Select All box
Click the Export icon in the top right corner of the screen to select Fullname and Email addresses to be exported in the CSV file.