All messages addressed to a virtual domain are routed through a sendmail mailer called /etc/mail/multihome. This program does virtual domain aliasing, then it converts any virtual domain user names to physical names via the user map, and finally it resubmits the mail back to sendmail for local delivery (and possibly forwarding depending on the results of aliasing).
There are several configuration files that are used and all are maintained by the Virtual Domain User Manager (see Configuring mail and messaging):
This file and /var/internet/ip/127.0.0.1/mail/virtdomains.db (the corresponding database file) contain a copy of the IP aliases that are currently configured on the machine. It is updated automatically by running the Virtual Domain User Manager after adding or deleting an IP alias.
This file and /var/internet/ip/127.0.0.1/virtusers.db (the corresponding database file) contain the virtual domain user map. All the virtual domains are in this single file even though the graphical user interface (GUI) presents the file one domain at a time.
These files (one per virtual domain) and their corresponding database files, (/var/internet/ip/ip_addr/mail/mail.aliases.db) contain the mail aliases for each virtual domain.
See multihome(1) and multihome(1M) for more information on the multihome program.