Make sure to set IMAP setttings to “Download complete item including attachments”. If this is not configured correctly you may see the "Message body unavailable" or "Empty". You can change the default settings in your Send/Receive settings:
1. Edit the Send/Receive settings
Press CTRL+ALT+S and select the Edit… button.
Another way of reaching this dialog is via:
Outlook 2007 and previous
Tools-> Options…-> tab Mail Setup-> button Send/Receive…-> Edit…
Outlook 2010 File-> section Options-> section Advanced-> button Send/Receive…-> Edit…
If you have multiple accounts configured, select your IMAP account in the left column.
2. Configure download behavior
How to configure the download behavior depends on your version of Outlook and your desired settings.
Outlook 2003 and previous
In the Folder Options section, select a folder and change the option from from “Download headers only” to “Download complete item including attachments”. You’ll have to do this for each individual folder; there is no way to change this for all folders at once.
Outlook 2007 and Outlook 2010
Under “Receive mail items” you can change the download method for all folders at once. If you want to control it on a per-folder basis, then you’ll have to select the option “Use the custom behavior defined below”.