All voice messages for Agent Washington will be left waiting for him in a private comm channel whenever he's able to get to them.
Any mail sent to him he'll retrieve upon his arrival to the Plane. It'd just be easier that way since explaining random letters arriving on a secret military spaceship/base would be a little difficult.
If you have any feedback or constructive criticism for how I'm playing Agent Washington, or have any questions at all about when I'm taking him from, etc, go ahead and leave a message here or email me!