It sends them an error message.
You can't have it post as guest... otherwise if a spam bot gets ahold of the email address your looking at a helluva lot of posts
Those who are familiar with mailing lists will know that you have to set one specific email address as the one your emailing from.

So this will not be an alteration to what they are used to
Also, note that the person's email address is hidden in the emails that are sent out. Unlike yahoo where everyone on the list sees your email address

So your users maintain the complete email address security that vbulletin offers even using this modification