I am trying to set up mailboxes for general campaign communications, and have followed your steps here: http://nationbuilder.com/how_to_use_mailboxes 

At the end of the steps it says "If you're using NationBuilder as your email provider, you're now ready to send and receive emails."

Great, but WHERE do I do this?  Please advise where to find the mailboxes to start sending and receiving emails via my NationBuilder email address. Thank you!