The Set as Default account feature in the app determines which of your accounts will be the primary one when you create a message.
To set the default account:
1.From the ☰ MyOffice Mail menu in the upper-right corner of the Main Window, select Account Settings.
2.On the Account Settings tab, select the desired account in the left pane.
3.In the Account Actions menu for the account in the left pane, select Set as Default.