[plug] remote site cannot send attachments

[Jon Miller]

I have a problem that I nor some of the vendors can solve and I'm hoping someone can shine some light on the subject.  The layout is a VPN network in a Hub-an-spoke configuration using Cisco 2621 at the hub and Cisco 837 and Cisco 1721 as spokes.  On the majority of the spokes the workstations are connected to a switch and the switch to the router.  The mail server is a Linux Debian server with postfix and Sophos MailMonitor on it.  The mail is scanned by MM and past on to Postfix where it is delivered to the mailbox. 
The problem is the following:
When a user sends a regular email from either Netscape v7 or OL2K it goes through ok.
When the same user sends an e-mail with an attachment it hangs and eventually times-out.
The last line of the capture is where we are puzzled.  The attachment is the ReadMe file from Radmin and the program only get halfway through it and then it has the 451 error on the line.  I've asked Sophos but they do not have a clue as to what is going on.
The routers on the spokes have a MTU size of 1436 and the hub router has a MTU size of 1492.
I've noticed that there is a packet that states 
[TCP Analysis Flags]
    [This is a TCP Duplicate ack]
[Duplicate ACK #: 1]
[Duplicate to the ACK in frame: 1284]
Not sure what this means except that it's a duplicate of the previous packet.  But the question is why?

I would appreciate any light someone and put on the subject. 
This is the content of a TCP stream from the capture. I've deleted about 25 lines of text to minimise the size of the capture.

220 SMTP Server Ready
EHLO gfp.com.au
250-gateway
250-SIZE 10240000
250-ETRN
250 8BITMIME
MAIL FROM:<test061 at gfp.com.au>
250 Ok
RCPT TO:<susanr at gfp.com.au>
250 Ok
DATA
354 Start mail input; end with <CRLF>.<CRLF>
Message-ID: <421D440C.5060501 at gfp.com.au>
Date: Thu, 24 Feb 2005 13:33:40 +1030
From: Test <test061 at gfp.com.au>
Reply-To:  test061 at gfp.com.au
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0 (BDP)
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: susanr <susanr at gfp.com.au>
Subject: test2
Content-Type: multipart/mixed;
 boundary="------------090205010000080802080408"

This is a multi-part message in MIME format.
--------------090205010000080802080408
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit



--------------090205010000080802080408
Content-Type: text/plain;
 name="README.TXT"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
 filename="README.TXT"

----------------------------------------------------------------------
 Remote Administrator 2.2 for Win9X/ME/NT4.0/2000/XP
―-------------- snip ―--------------------------------------------------
File Transfer
Current version lets you transfer files (less than 2Gb) between
computers  with Windows Explorer like inter451 MailMonitor for SMTP: Processing error occurred, try again later
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