If you send whoever a message, then delete the user name and insert another persons name it should not work if they are not in the inbox system.
In order to be in the inbox system they must have previously logged into the community website (community.roll20.net). Logging into the application website (app.roll20.net) does not get you into the inbox system.
Again, this is one area that is being looked at for the current development cycle. The hope is that a message system will be created that works across both websites.
- Gauss