Horde imp header format

Horde Internet Messaging Program (IMP)

Message-ID: <1186073825.46b20ce1ad3cd@sendinghost.com>
Date: Thu,  2 Aug 2007 20:57:05 +0400
From: username@sendinghost.com
To: username@receivinghost.com
Subject: Header test
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-5
Content-Transfer-Encoding: 8bit
User-Agent: Internet Messaging Program (IMP) 3.2.2
X-Originating-IP: 255.255.255.255

Message-ID format:

horde/lib/Horde/MIME.php
...
    function generateMessageID()
    {
        return '<' . date('YmdHis') . '.' .
            substr(base_convert(microtime() . mt_rand(), 10, 36), -16) .
            '@' . $_SERVER['SERVER_NAME'] . '>';
    }