Mail Server Design

[Mariano Wahlmann <wahlmann@agro.uba.ar>]

I need some help, on designing a mail server solution (structure), my
request are:

-Users 4000+ aprox. with a growth rate of 20/users by month.
-Each user has 15Mb of inbox.
-Accesible by POP3 / IMAP (Only for Webmail who resides in other server)
-Mail list (some of 4000+ users)

I need a design solution for the server in wich resides mailboxes.

I have two proposal:

1- One big expensive server with 1 SCSI disk, two processors.
2- Several cheaps servers, dividing mailboxes across servers.

which is best solution?

Thanks.