68305 Error Page

I have a webmail service. I created a branded 1st page for the service with a php script which allows people to login. The script works great, but when an incorrect domain is entered, it only gives a page connot be found error. You can see an example here: [login to view URL] If you enter an incorrect domain such as: [login to view URL], you get the correct login page If you enter an incorrect domain such as: [login to view URL], you get page connot be found. I would like to customize an error page and have the customized error page replase the "page connot be found message" Can anyone help with this?