Mail Software Used by ISPs

I am interested in information on what software small, medium, and
large ISPs use for email services. When I refer to "email services" I
mean SMTP, POP, IMAP, web mail interfaces, and the back-end
administrative tools. Do many/most ISPs use big, expensive enterprise
tools for this? Piece their own systems together using various
commerical/free tools for the different parts? Or do they roll their
own almost top to bottom?

I have experience doing email for small and medium corporate sites (in
the 100's of internal users), but the particular project I am working
on is for customers, not internal users, and could quickly balloon
from the few dozen on the current demonstration system to thousands.
Any firsthand info or pointers to more information would be greatly
appreciated.