[meta] List should be a little bit cleaner from now on

[Santiago Vila]

Hi.

This is to tell you that messages having a x-dnsbl-warning: header
will not be distributed to the list anymore (thanks to Robert
for allowing me to do this change in the list configuration).

So please make sure that you do not post a message from a mail
server in sbl.spamhaus.org, otherwise the message will be held
for moderation (and we can't guarantee to be 24/7 available to
moderate posts).

Hopefully this will make a lot of the recent spam to disappear.