login on https://outlook.office.com > click on settings (*) > select Mail
Now you are on outlook settings page > select Layout under Mail > select Message format > select Always Show From > Save the settings > & close the settings page
Now click on Options (You will be back to your Inbox now) Select New > Email Message > select more (...) button from menu > select Show from > now from button is enabled.
Now from option is enabled on your screen.
Now right click on existing email address & click on remove. Now add new email address which you want to use.
To Being with
1. To start using your mail on new platform, you need to log on to
https://portal.microsoftonline.com/
OR
https://portal.office.com/
https://portal.office365.com/
Enter your full Email Id and the Temporary passwords given. There is separate initial password for this site.
(NOTE: If you haven’t received or misplaced initial password, please contact IT department)
2. You will be prompted to a screen like this:
3. Now you can select App which you want to access