SharePoint Maintenance Page

Three options for the SharePoint Maintenance page based on the available infrastructure:

1)      Using modified ISA custom error page and display custom messages.
          - For internal clients, the files are named Error_number.htm. \
          - For external clients, the files are named Error_numberR.htm (where R indicates reverse).

2)      Display custom message on IIS at Web Front End Server in the SharePoint Farm
         - Create a page "App_Offline.htm" in the Root Site

3)      Route the request from ISA to another Site hosting maintenance page.
         - Maintenace page site on same IIS

Reference: ISA