Margaret,
If that is on our hosting, webmail is by https, and if your domain does not have a valid cartificate (and a dedicated IP address needed for a SSL certificate) you will be getting that warning.
To not get a warning, use the alternate webmail address as received in your setup email. Login is same.
If you have any questions, please login on our hosting portal and open a ticket there, so that we can help you with exact information (as it depends on your hosting).
Jonathan,
I just saw your post above, but I think i saw same in our hosting ticket support, around a month back too and believe it is solved issue. If not, please open a ticket on our hosting. It is a security feature of webmin, and by adding an alias virtual server with another domain name, you can access fully.