[ILUG] deliveries to postmaster Sendmail securiety issue

[Kieran O'Sullivan]

I have inherited a server with sendmail setup on it.  There are a lot of 
messages coming to postmaster stating that addres at somedomain.com does not 
exist.  I am certain that noboey on the server is sending these messages 
out so I'm not sure if there is a securiety problem with the sendmail 
setup or if this server is just getting blowback from someone slese SPAM 
activity. 

Can anyone tell me why postmaster would get bounc backs for e-mails which 
have not been sent out?
What kind of tests can I do to see if sendmail is no secure?
Thanks