[OT] spam and sendmail configuration

[viktoras d.]

Sorry, this is quite OT, just I am desperately seeking an advice. Could 
anyone please suggest a good resource on sendmail access file 
configuration with exhaustive samples on fighting spammers (I googled, 
found something, still looking for the "best" one). A few days ago I 
started receiving lots of  spam with forged FROM address as if it was 
sent from my own domain, which I think should be impossible because my 
sendmail is configured to NOT relay anything, and I am using ISP's 
mailing server to send my mails which is secure, password protected, and 
so on... Tested my domain with 
http://verify.abuse.net/cgi-bin/relaytest, it says all tests performed, 
no relays accepted. Still I was receiving spam from "myself" and other 
"domain users" each time with different name... Yesterday configured 
sendmail's access list to reject any of those "alien names", and they 
stopped appearing in my inbox, but today I received twice as much spam 
from other sources and also from "myself". So, I would appreciate any 
good advice on how to best deal with this problem at the mail server (on 
Linux) level.

Is it possible to define more complex rules in the sendmail's access 
file ? I know about blackhole service, just looking for other options.

Best wishes
Viktoras