Spam Control Considered Harmful

Jon Lewis jlewis at
Wed Oct 29 05:55:37 UTC 1997

On Tue, 28 Oct 1997, Jordan Mendelson wrote:

> I personally do spam filtering for our site. Actually, it's not "spam" 
> filtering per se. If you don't have a domain in the from addr which resolves, 
> your mail is rejected. If you are not a customer of ours and try to relay mail 
> off our servers, your mail is rejected.
> This to me seems completely just. Why should you send mail with a false return 
> to address and why if you are not my customer should you send mail?

These are standard features to most sendmail anti-spam/anti-relay patch
sets.  Now, what about blocking mail if it's passed to you by a host that
has no record?  I've recently started doing this on a few
systems since I've found that some spam providers (either because they
move too frequently, don't want to be resolved, or just don't have a clue)
don't have reverse DNS. 

I'm blocking several hundred messgaes/day per system and get log entries
such as:
sendmail[1725]: Ruleset check_relay ([], 
rejection: 418 obtain a hostname

So far, I've gotten no complaints, so I assume nearly all the mail that
can't get in is junk mail.

> Now, filtering based on hostname & blackholing is a bit extreme. It limits the 
> user's right to choose. As long as the commercial soliciter has a valid 
> reply-to address which you can use to bitch and complain, then I feel it's 
> fine.

What about valid (i.e. resolvable) from addresses that are invalid for
mail delivery?  i.e. if you get a lot of spam, surely you've gotten
messages from who knows where, claiming From addresses like
897632 at  Sendmail rules will resolve that, but email a complaint
there, and it's likely to bounce.  I've not figured out a sendmail rule
for blocking such mail from: addresses.

 Jon Lewis <jlewis at>  |  Unsolicited commercial e-mail will
 Network Administrator       |  be proof-read for $199/message.
 Florida Digital Turnpike    |  
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