I'm not sure howto discover your problem using that log
What I do?
I see the header of one message looking for info like this:
Return-Path: <jader@linuxfacil.net>
Delivered-To: jader@pantera.roan.com.br
Received: (qmail 8793 invoked by alias); 2 Jan 2010 10:19:15 -0000
Delivered-To: alias-localdelivery-jader@roan.com.br
Received: (qmail 8790 invoked by uid 453); 2 Jan 2010 10:19:15 -0000
X-Virus-Checked: Checked by ClamAV on roan.com.br
X-Spam-Status: No, hits=0.0 required=5.0
tests=BAYES_50,SPF_PASS
X-Spam-Check-By: roan.com.br
Received: from outbound-mail-24.bluehost.com (HELO outbound-mail-24.bluehost.com) (69.89.21.19)
by roan.com.br (qpsmtpd/0.40) with SMTP; Sat, 02 Jan 2010 08:19:09 -0200
Received: (qmail 7645 invoked by uid 0); 2 Jan 2010 10:19:03 -0000
Received: from unknown (HELO box544.bluehost.com) (74.220.219.144)
by outboundproxy2.bluehost.com with SMTP; 2 Jan 2010 10:19:03 -0000
Received: from 189.27.174.221.dynamic.adsl.gvt.net.br ([189.27.174.221] helo=lobo.linuxfacil.net)
by box544.bluehost.com with esmtpa (Exim 4.69)
(envelope-from <jader@linuxfacil.net>)
id 1NR14Z-0003sQ-2P
for jader@roan.com.br; Sat, 02 Jan 2010 03:19:03 -0700
Received: (qmail 13442 invoked by uid 453); 2 Jan 2010 10:18:56 -0000
X-Virus-Checked: Checked by ClamAV on linuxfacil.net
Received: from localhost (HELO localhost) (127.0.0.1)
by linuxfacil.net (qpsmtpd/0.40) with ESMTP; Sat, 02 Jan 2010 08:18:55 -0200
Received: from macmini.linuxfacil.net (macmini.linuxfacil.net
[192.168.13.204]) by lobo (Horde Framework) with HTTP; Sat, 2 Jan 2010
08:18:53 -0200
Message-ID: <20100102081853.16424v6jtirseyec@lobo>
Date: Sat, 2 Jan 2010 08:18:53 -0200
I do this to discover if the problem was internal to my server (delay to try to delivery) or something else (like antispam rules on ISP or target server).
If your problem is just for one destination... I bet on spam (or antispam) rules.
What a header of one delivered message searching for this info and watch your smtp log to see an delay caused by antispam rules (like a tarpit).
I setup two autoreply accounts (on inside my country, in an account on biggest ISP here) and another on a hosting service I use inside USA. I use them to diagnose e-mail problems.
You can try them: respostaautomatica@terra.com.br and respostaautomatica@abandonemicrosoft.net
but try BOTH of them in EACH message... if you get no answer... that's a bad signal.
BTW: the autoreply messages are in portuguese... try google translate them if you're curious!