Sendmail/LogWatch reports (may be forged)

[Timothy Alberts]

Greetings,

I am running a FC4 sendmail server and I've been trying forever to at
least limit some of the spam.  In this effort, I have been adding to the
Access control (/etc/mail/access) domains that are known to be mail
bombing my domain.  A few continue to evade the sendmail filtering and
are still getting through.  I know this because LogWatch reports:

Unknown Local Users
  invaliduser at mydomain.com
    from *.speedy.net.pe ... (may be forged)

where * contains the specific client that continues to change.  My first
attempt to block them, I added to /etc/mail/access 

speedy.net.pe	REJECT

to try and reject the problem domain.  This doesn't work because
LogWatch continues to report to me that mail is coming in.  I've tried
to reject on the IP as follows:

201.230.19.113	REJECT

but of course, they just changed IP address.

Can anyone explain to my the whole in my security that is allowing them
to get through and how to plug it?

Thank you in advance.