Office 365 (Outlook 2013 / 2016 / 2019 for Win) - Changing the Account Used to Send Email Messages
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.