[ILUG] sendmail and filters

[Barry Redmond]

(With DIT hat on)
A smallish mail server that I manage is seeing a lot of junk mail of a 
particular form coming in, and I want to try to block it without spending 
days wandering lost in sendmail's config files.  I once understood a 
little bit about sendmail, but those brain patterns decayed a long time 
ago and I have no inclination to train replacements.  Now I just turn 
sendmail features on and off.

The junk mail comes from different sources because it's being 
"returned" from non-existant destination addresses.  The return 
address is always a real address (different each time) on one of our 
systems, but with 6 apparently random digits appended to the 
username.  Is there any *easy* way to insert a filter that can apply a 
regular expression to parts of a header as the mail is coming in, and 
give a reject/accept decision?  

I already use a realtime blackhole list, and block some specific junk 
addresses, but this case needs a finer mechanism.

Thanks,
Barry Redmond