This body part will be downloaded on demand
There seems to be some problem in the way the Thunderbird email client handles certain kinds of attachment which means the message "This body part will be downloaded on demand" is displayed when you try to view a message. We've seen this problem both with VCard attachments (.vcf) generated by Thunderbird when combined with other attachment types, and with certain attachments sent from Microsoft Outlook.
If you use POP3 to fetch your email then you can just download the full message and it displays with no further problems, but if you use IMAP (which many of our customers do) then there doesn't seem to be a way of downloading the body part of the message.
Luckily there is a simple workaround which I found at mozillaZine. The solution is to tick the "Display Attachments Inline" in the View menu, which causes Thunderbird to download the entire message along with all the attachments and display the attachments that are viewable (like images) in sections below the main message body.
Easy when you know how :).