given your citadel may recieve mail for two domains:

• acme.com
• mycitadel.com

and now you want sales@ to be delivered into different rooms you could do this the following way:

• create a room acme_sales
• create a room mycitadel_sales
• add sales@acme.com,room_acme_sales to your mail.alias file
• add sales@mycitadel.com,room_mycitadel_sales to your mail.alias file

from now on these two email adresses will become delivered to different rooms. You could imagine the same scenario for two users, like

• einstein_jr
• einstein_sr

is here. Now Junior should receive einstein@acme.com, and Senior einstein@mycitadel.com. This will be just possible if there is no einstein user! Now Junior will have to add einstein@acme.com to his personal v-card, Senior adds einstein@mycitadel.com to his. Both will now receive the mail accounts they like.