It might refer to a embedded image stored in another MIME section, after the message body. The filename the image originally came from is mentioned in the headers, but its not an attachment. For example, I have a HTML message with a embedded image that uses
<img alt="" src="
cid:part1.09020004.05030809@yahoo.com height="1200" width="1600">"
in the message body to refer to the embedded image at :
--------------070407040009020807040302
Content-Type: image/jpeg;
name="58919_1600x1200-wallpaper-cb1346964996.jpg"
Content-Transfer-Encoding: base64
Content-ID: <
part1.09020004.05030809@yahoo.com>
Content-Disposition: inline;
filename="58919_1600x1200-wallpaper-cb1346964996.jpg"
/9j/4R2sRXhpZgAASUkqAAgAAAAOAAABAwABAAAAQAYAAAEBAwABAAAA6wQAAAIBAwADAAAA
tgAAAAMBAwABAAAAAQAAAAYBAwABAAAAAgAAABIBAwABAAAAAQAAABUBAwABAAAAAwAAABoB
BQABAAAAvAAAABsBBQABAAAAxAAAABwBAwABAAAAAQAAACgBAwABAAAAAgAAADEBAgAgAAA
..... (the big block of characters is the actual image data, base64 encoded)
If you're running into something else it would help to also see the HTML statement that references the cid, and a few of the neighboring lines above and below the reference to the mailbox. I normally see a mailbox reference only in the properties (in the folder pane) for a mbox file such as the inbox in Local Folders :
mailbox:///C:/Users/Eric/AppData/Roaming/Thunderbird/Profiles/feu8hx1l.default-release/Mail/Local Folders/Inbox