As many users have noticed, Outlook on the web, including both Outlook.com and Office 365 business accounts, changed the sort order of Contacts in the address book that comes up when you click the To or CC button.
The contacts are in a weird sort order – supposedly most frequent contacts – but one that makes little sense to many users. There is no search or filter capability in the Address book, and the sort order can't be changed.
This is the explanation from Outlook.com Support:
When you access the contact list from the To/CC/BCC area while composing a new email, the list that comes up will be sorted on the basis of how frequently you have been in a conversation with different people. The people you have sent most emails recently will be on the top. This feature was introduced to enhance the efficiency of working on outlook.com.
I'm not sure whose efficiency is improved, certainly not users who can't find less frequently used addresses.
If you want to see the contacts in alphabetical order, you'll need to open People to select the contacts. You can select multiple contacts in the People module, then click Send email to address a new message to the selected contacts.
If you don't like the new sort order, please vote for and comment on this suggestion at Outlook.com Uservoice:
I am not able to check my contact list alphabetically while sending email by clicking on new email