[Exim] re please help !!!!!

Please Help Me out .I am new to the World of exim /UNIX/Linux

Our companys mail set up is as follows .
Our enterprise is a multilocation multi divisional setup spread across
various parts in india.
All the employees in the enterprise are using ccmail so the inter ofice
mails are exchanged thro ccmail.where all the divisions and branches have
ccmail servers which exchange mails thro a central mail server .For the
users who are moving around frequently another mailing system is devised
which is provided thro netscapemail server.all the ccmail and netscape mail
users use one commain domail to relay the mails .the ccmail gateway is
mapped to the netscape mail servers domain and all the mails to be sent
outside are also sent thro' it.
Since we donot want to upgrade our netscape mail server to the latest verson
4.0 we use (ver 2.0) we would like to cofigure exim as our MTA.Due to
security reasons.
Is it possible to do such a change.
How difficult it would be to bring about such a change without causing a
significant down time .(we have approx 4500 mail users in all)
Is it possible to configure such a thing on linux.
Do all the users of netscape have to be transferred to exim .If so what
about the mail data.How long would it take to configure .Our consultant says
10 days (is it that easy .)
Would we able to transfer all the netscape users on exim without any changes
in their configuration setup.
Can ay one briefly explain me or suggest me some solution or some site which
is giving detailed info
Ps: note I am new to unix /linux answer considering I am a new member
joining the unix /linux group still learning it .
Can any one explain me does exim 3.16 have anti spamming facility Ie
blocking the spammer s mails auto matically.?????
PLEASE HELP ME OUT
DO TELL ME THE APPROX TIME FRAME THAT THIS CHANGE CAN BE BROUGHT ABOUT >>
Hoping for some reply ..
Regards
Manral Neeraj .