[ADMIN] Anti spam measures

Torbjorn Granlund tg at swox.com
Sat Feb 19 22:00:05 CET 2005

The new anti-spam mechanisms seem to be effective; only one
spam has reached the moderators after the lists were reopened.

We have implemented two new anti-spam mechanisms.  It is good for
list members to be aware of them.

1. The SMTP client that contacts the swox.com mail server must
   have PTR records in the DNS.  (I.e., there must exists a
   mapping IP-to-name.)

2. Messages from all SMTP clients are greylisted, and initially
   rejected with a 450 code.  Such a code means "error, but try
   again".  Almost all spam today come from cracked Windows
   computers, using simplistic SMTP clients.  These clients
   typically don't use a proper mail spool, but just give up when
   we tell them to retry later.  RFC-compliant SMTP clients will
   try again, and if they retry, our server will accept the
   message.  (If spamware actually retries, its IP address
   typically will have gotten listed by the exploit block list we
   use, xbl.spamhaus.org.)

Note that the greylisting will cause some delays before a message
is accepted for distribution to the list.  In some cases, where
your site have many load sharing SMTP clients, the delays can be
hours or even days.  We have experienced such delays with
gmail.com.  The greylisting is based on exact IP number, but
gmail.com uses a different SMTP client each time they retry!

If you have problems getting your messages to the lists, please
first make sure your SMTP client have a PTR record.  If it does,
the message should ultimately get delivered to the list.

I regret the inconvenience these anti-spam mechanisms might

More information about greylisting can be found here:

If you have questions, please contact me privately.  Do not reply
to the lists.  If you cannot reach the lists, you will not be able
to reach me either, unfortunately.


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