Courier and DCC

Vernon Schryver vjs@calcite.rhyolite.com
Wed Apr 10 17:37:57 UTC 2002


> From: Vincent Schonau <vinces@xs4all.nl>

> >> There are actually two phases to filtering: rcptfilters are called
> >> after you have the HELO/MAIL FROM/RCPT TO, and you can give "thumbs
> >> up", "thumbs down", or "maybe" at this point.
>  
> > How do they handle multiple RCPT_TO values?
>
> When a recipients mailfilters reject the message, an appropriate 5xx
> response is issued during the SMTP conversation. If some recipients want
> to accept, but others don't, and the sending SMTP client continues after
> this 5xx error (with DATA), the mail will be delivered to the address
> who had accepted the message, but not to those that hadn't.

What about individual recipients who decide they don't want the message
at the end of the DATA command?  That's the situation with the DCC.
It is not until the end of the DATA command that recipients know that
they don't want a message because it is bulk and something about the
message (e.g. the sender) is not whitelisted.

My tactic with dccm+sendmail when there are both willing and unwilling
recipients is to use the sendmail milter interface to tell sendmail to
remove those who don't want to receive the message from the addressee
list.  That has the disadvantage of not generating a bounce for those
addresses that refused the message, but it does deliver it to those who
want it.  The underlying problem is that the SMTP server can only say "2yz
good message for all recipients" or "[45]yz bad message for all recipeints"
in response to the DATA command.  (I wouldn't count on a 4yz response
to the DATA command working with all SMTP clients.)


> This seems appropriate; it works with my installations. I'm not quite
> sure how mailservers other than qmail respond to seeing 5xx during the
> SMTP-conversation, especially before DATA.

Historically, 5yz answers to DATA have been the problem.
For example, Hotmail long ignore 5yz responses to the DATA command,
and now, leaks potentially private information from other transactions.


> There's a blurb in the localmailfilter documentation that refers to
> 'users being unsubscribed from mailing lists due to other users poorly
> written filters'. 

No reasonable SMTP would be confused by a 5yz response to one Rcpt_To
command.

There are words in RFC 2821 about 5yz and 4yz responses to individual
Rcpt_To commands.


> >> The one thing I didn't see was the HELO value!
>
> The HELO value is reproduced as
>
>  Received: from <helo-value>
>
> in the top Received: header, available to smtpfilter.

That seems a little late for filters other than the DCC, because
most of them want to filter before the DATA command and so before 
any of the headers have been received and so before a Received header
can be added.

For filters like the DCC that don't do regular expressions, it is
not helpful.


Vernon Schryver    vjs@rhyolite.com



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