Documentation of big "mail systems"?

[Stephane Bortzmeyer]

I'm currently writing a proposal for a webmail service for, say, 50
000 to 500 000 users. I'm looking for description of existing "big
mail" systems, using technologies like scalemail
(http://scalemail.sourceforge.net/), specially with an emphasis on the
storage subsystem for the servers (my weak point, I don't really have
enough experience with SAN, NAS, and so on).

Of course, with a FreeBSD (and free software) bias :-)

I do not need general advice (such as "Postfix rules, Exim sucks" or
"Maildirs are faster") but actual description of existing and running
systems. Googling seems inefficient for that purpose and I presume
that many interesting papers are only in closed and paying conference
proceedings :-(