On Microsoft Outlook, when you have multiple email accounts, the first account you configure during the initial setup will dictate many of the default settings. When composing a new email, the app will use the address of the mailbox you're viewing as the default to send the message.
However, if you tend to use a different account to send messages, you can set that account as the new default to create new emails and the default account the desktop version of Outlook should open every time the app launches.
