My users are to have addresses in 3 domains. Using same username (before-@ string), same mailbox, but different domains (after-@ string). I've created all the users in one domain and aliases in another. But this only works for incoming mail, while users must be able to send outgoing messages choosing whatever domain to be specified as sender (and it'd be great to set different default domains for outgoing messages and use the same domain to which an incoming message came if answering a message). How can I achieve this?