Citadel supports multiple email addresses for a user, and they may be in different domains. To do this, you must have the MX records for all domains pointing to your Citadel server, and then identify the domains to Citadel: Administration -- Global Configuration -- Domain names and Internet mail configuration -- Local host aliases.
Then edit the user: Administration -- Add, change, delete user accounts, select the user to be edited, followed by "Edit configuration".
"Primary Internet e-mail address" is the address that will be used for outgoing mail. The user will also be able to receive mail at this address. "Internet e-mail aliases" will contain all other email addresses for the user. These addresses may be in any domain that is valid for your site.