Junkmail alerts

Does anyone know if it is possible to custimize the junkmail alert which is emailed to users? The link points to webmail.dreamhost.com requiring the full email address, when my users are used to only having to type in their login at webmail.mydomain.com

This feels like a silly request, but for some reason people just can’t figure this out.

edit: I know about changing the login page for webmail.mydomain.com, just not the email alert.