From 
From: Author <author@example.com>
To: Recipient <recipient@example.com>
Subject: Sample message 1
Date: Wed, 22 Aug 2005 17:20:08 +0000

Earlier versions of Thunderbird don't seem to quote things like this:
From 
This could cause problems, if a reader decides to split the message
here.

From 
From: Author <author@example.com>
To: Recipient <recipient@example.com>
Subject: Sample message 2
Date: Thu, 23 Aug 2005 11:17:43 +0000

This is the second body.