From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rakotomandimby Mihamina <mihamina@mail.rktmb.org>
Subject: Re: Blocking mail relay from the firewall?
Date: Thu, 15 Jul 2004 19:26:52 +0200
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Default port for that is 25.
If you wanna block that traffic, just drop whatever is from and to port 25.
But the best way is still to configure your MTA (exim, sendmail, ...) 
server.