Well, it requires a bit of 'smoke and mirrors', but it's doable. First, pick a primary Gmail account and set the others to forward all received mail to the primary and to delete from the original account. Do that from the web interface. That will bring all mail to one inbox. To send new messages, you could use templates or separate folders for the various accounts, setting each with a different email property. However, if this is a key requirement for you, then you may wish to evaluate Thunderbird or Postbox or Operamail (not sure if that is still supported), all of which do what you are requesting, in that they provide the transparent impression of a single inbox and outbox. Good luck.