In the public address book, there is a view called Domains; it’s under Configuration, Messaging. In there, you should have a Global Domain document. In that document, on the Conversions tab, there is a “Local primary Internet domain” field. Yours probably says acme.com. Below that, there is a “Alternate Internet domain aliases” field. In there, you should put planes.com and trains.com That will take care of the need to accept mail addressed to any of those domains.
As for outgoing, the only thing that comes to mind is to set up separate location documents for each of the domains. Create a location doc called Planes. Set the “Internet mail address” field to Jim@planes.com Set up other location docs for the other domains. The user would switch to the Planes location to send mail from Jim@planes.com. and then switch to the Trains location doc to send mail from Jim@trains.com, etc.