If a contact has more then one email address but you want to use one as the default then the way to do this is to select the radial button against the email address that is to be used as the default email.  

Please see a step by step guide on how to do this:

1. If a contact inside your CRM account has more than one email address such as the example shown below you can select which email address is the default email for use when sending the contact e-Marketing campaigns or calendar event email reminders.

2. Open up the contact record and click on Edit as shown below to open up the contact details:

3. The email addresses are shown depending on which one is to be the default email address for the contact you will see a round button which you can select by clicking your mouse in the round circle - as shown below in red the circle is blank:

4. If you wish to make one of the email addresses the default email address then just click inside the circle as shown below:

5. Click on Update to save your default selection.

6. Please note: the same can be applied for phone or mobile numbers.  

If the contact has more than one number they can be contacted on then you can set up the preferred contact number by using the radial button against the number that the contact prefers to be contacted on.