From the traditional Outlook email client

Open a new email.

Go to 'Options' on the ribbon and click 'From' in the 'Show fields'.

You will then see the 'From' field about the 'To' and 'Cc' fields.

Click the Drop down arrow on the 'From' field button and select 'Manage list'.

Enable any additional addresses you wish to use:

Click the Drop down arrow on the 'From' field button again. You will now see the additional email addresses in the list and can select it to use for the current email.