The system messages are stored in Exchsrvr\bin\MDBSZ.dll
To be able to edit this file you can download RLTOOLS.exe from:
ftp://ftp.microsoft.com/Softlib/MSLFILES/RLTOOLS.EXE
(Included in this toolkit you’ll fin a tool called RLQuikEd.exe that will allow you to open and edit the dll.)
MDBSZ.dll is replaced in service packs so after upgrading the server you will need to make the changes again.
Changing the MDBSZ.dll is NOT supported by MS PSS.
But if you contract MCS to do the changes for you, the MCS will support the changes (Not PSS)You can also use Exchange Quota Message Service to do generate custom quota messages for users on Exchange server without editing DLLs
More information about Exchange Quota Message Service:
http://www.gotdotnet.com/workspaces/workspace.aspx?id=b82dabfa-8353-472c-b82b-1423263ab1bb