How to trap SMTP events of a SMTP server

Hi,

I would like to monitor the each incoming mail for all my mailserver users (1000 users)..& accordingly update my database…
The mailsever has got an SMTP server & POp3 server..
I guess i need to capture the SMTP arrival events on the SMTP server to get the info on incoming new mail…
for
1>Linux Mail server
2>Exchange server 5.5

Are there any APIs availables to do this..?
or should i keep polling thru all the mailboxes using POP3 to check for
new mail…?but this looks not asefficient a design as capturing SMTP arrival events on SMTP server..

Can u help me out in deciding the best design..!
& can you show me some sample code..

Thanks,
Nans