Fw/etc/vdomain.cw
In order for email addressed to virtual domains to be accepted, each of the virtual domains must be added to the $w macro. The /etc/vdomain.cw file holds them in the form of a simple list, like so:
virtual.com custom.net
This is exactly the same as the /etc/sendmail.cw file, and does the same thing. If you maintain your /etc/sendmail.cw file "by hand", you can simply list your virtual domains in it and skip this file.