filter unwanted domains?

[John Van Ostrand]

On Fri, 2006-09-08 at 15:56 -0400, Chris Aitken wrote:
> I get a lot of spam.

How much is a lot?

> Is there a way I can set a rule or set up a filter to send messages to 
> senders that they need to email me again and have me accept them? Then I 
> could add their domain to my friendly-domain list or somesuch. So, for 
> instance, I could add ***@dsb1.edu.on.ca as friendly domain, or as I'm 
> dealing with Sears now I could add sears.ca - that sort of thing...

If you have your own mail server you can use greylisting. The technique
is simple. It refuses email the first time it is sent and forces the
sending server to retry later on. This is based on the fact that most
spam servers are not persistent. If they fail to deliver a spam on the
first try they simply move on.

Greylisting works by keeping track of the sender-recipient combination
of the email. It refuses the email only after it has this information
and if the combination wasn't recently seen before. The combination is
then stored in hopes the remote server returns. Legitimate mail servers
will return to deliver it at a later time. Some will wait 15m, busy
servers may wait 1h, it's configurable by the administrator. When the
remote mail server tries again your email server checks for the
sender-recipient pair and, now that it exists, it allows it. This pair
is then kept for a longer period of time so that subsequent emails are
allowed through on the first attempt.

The main downside that I see is that you will have to wait for the first
message that you get from someone. It also means that you have to
accommodate other special cases if needed. For example you may have some
web applications (fill-in-forms) that bypass mail servers and hit your
server directly. Or you may also have remote users who use SMTP port
from their mail clients. 


> Obviously I don't know what I'm talking about. However, I've received 
> returns to my emails where an organisation has done this - then I email 
> them again and they add me to some list so subsequnet emails will be 
> accepted.
> 
> I don't what this is called - I don't even know what words to google to 
> read up on this.
> 
> I'll pay a little (?) if I *must* but if there is something easy I can 
> do in Thunderbird 1.0 then I'll do that.
> 
> Sincerely,
> 
> Chris
> --
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-- 
John Van Ostrand
         Net Direct Inc.
 
Chief Technology Officer
564 Weber St. N. Unit 12
   Waterloo, ON N2L 5C6 
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john-Da48MpWaEp0CzWx7n4ubxQ at public.gmane.org
        Ph: 519-883-1172
 ext.5102
Linux Solutions / IBM
Hardware
        Fx: 519-883-8533
 

--
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