How to manage and add mailboxes?
Open your profile from the top right of the screen using the profile icon with your initials 
Now select 'Mailboxes' from the drop down menu
Choose which mailboxes to view.
Click into the sliders 
to to select what to display and share.
...
You can select a mailbox as a primary mailbox by clicking on the mailbox as below and selecting '
'
Your new email will appear as below with your from email entered automatically
Add more mailboxes by returning to the 'mailbox' screen and clicking 
You can now enter SSO details or configure manually.