Sendmail and user accounts

[Rushan Sobar]

Hi,

I'm running a fedora core 4 box and recently I notice spam and junk e-mails passing to user's that not shown to public and hard name user's that hard to be guess by dictionary or e-mail generator, I would like to ask is there any way that some can figure or scan for active user names on my box , is there anything that I should check to be sure than no one can extract active username of e-mails?

Regards
Rushan