It is sorted alphabetically but it depends on how you have added your contacts details and how you have them sorted in the list. It can be by first name or last name. You should check the contact details are entered correctly.
Go to your contact list and click on up and down arrow in the menu bar at the top of the list and sort it how you prefer.
The address book is alphabetical. However, when sending emails and the CC or BCC boxes are ticked, the addresses do NOT appear in the same format and are totally random, in email addresses, Christian name or Surname.
