If you're using Outlook desktop software (Premium subscription to Office 365), then you may want to specify which address book Outlook should use when you are sending emails. Here's how...
Hold down the following keys: Ctrl + Shift + B (this will open the Address Book)
Select the Tools menu, and choose Options.
Choose the address list that you want to use by default (this will usually be Contacts*), then click OK
*The Contacts address list is the one that synchronizes with your personal phone/tablet and personal online email.
Whenever you next create a new email and click to the "To" (or "CC") button, your chosen list of contacts will appear.