Ringtones are understood as being associated with your phone "as in ringing".
It sounds like your talking about "notifications" via when an email comes in, an you want to be able to customize a different sound for each person that sends to you?
This is a totally separate function from ringtones.
To the best of my knowledge Notifications of email are global.
I'm new to pda and smartphones... Im not aware if that feature is avail on any phone.