With this milter enabled (and sendmail.cf configured to use it), sendmail checks with the milter to learn if the user exist (or an alias). If not, the mail is rejected immediately.
If the vdomain has a fallback destination configured, the milter let the mail go.
An init script called vdomaincheck is delivered to start/stop the milter.
gives access to a form requesting the email address (not just the user id). From the email address, the form operates on the proper vdomain.
Here is how this works. When someone edit a page in Linuxconf using the HTML interface, Linuxconf sends HTML fields to the browser. It sends visible and hidden fields. The hidden fields contain the current status of the form. When the user hit "accept", the hidden fields are sent back. Linuxconf compares the hidden values with the current value in the configuration file being edited. If they do not match, it means another user has edited the same record. Linuxconf then reject the new input since it was based on information now outdated by the second user. Not doing so would be some information would be lost. For example
Triggering the bug was relatively easy. Access a form in Linuxconf and fill it with some errors. Linuxconf will warn about the error and let you correct it. When you hit accept, there is a good chance you see the "dialog mismatch" message.
Anyway, should be all right now.