How to check for existing mail accounts?

[Johnny Stork]

I have noticed a large increase in the amount of spam coming in to MS 
(latest) running on CentOS 5 and many are coming into non-existent email 
accounts. Is there a check that can be done for the existence of an 
account first, and then if non-existent, block even before any scanning 
is done, let alone processing through MS.

Thanks for any suggestions that anyone can give