And how can we help you with that? If your organisation doesn't have emails for your Moodle's Users, not a lot we can do about that.
Seriously suggest you change default home page to the Front Page and put up a notice all Users are to edit their Profile, click on their name and select Profile, then enter their actual email....
Go to Administration > Site administration > Appearance > Navigation and change the default home page to Site. Got to Administration > Front Page > Front Page Settings and change the Front Page and Front Page Items when logged in to None. Select the Include a topics section and then Save Changes.
At the Front Page click Turn Editing On and in the now available Topics Section add a label with whatever message you want in it.
Sure there is other ways to do it, but this is simplest I suspect. Dashboard is still available by clicking on their name.