Office 365 (Outlook Client for Windows) - Change the Account Listed in the From Field
If you have multiple accounts in Outlook, you can change which account is used to send a message. These instructions can also be used if someone else has has made you a delegate for their account and have set the permission levels to allow you to send messages on their behalf.
1. Open up a new email or reply to a message.
2. In the message window, above the To button, select From.
3. Select the account that you want to use.