If you find that email addresses are not set in a client mailshot, you need to ensure that the primary email address in the client record is set. Do check this, please follow the below steps:
Click Clients and then select the Client
Click the Phones tab
Make sure there are valid eMail addresses listed - if not add one
Click the Details tab and then click the pencil icon next to the Primary Email field
Select the email address from the list shown and click the tick icon