HTMLBody is supported in all versions of Outlook beginning with Outlook 98. Starting with Outlook 2003, it is considered one of the properties that may trigger a security prompt (depending on the code environment) when code attempts to read value. No prompt occurs when you set its value.
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Sue Mosher, Outlook MVP
Outlook and Exchange solutions at http://www.slipstick.com
Author of
Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers
No, it means that if you try to programmatically read MailItem.HTMLBody
property in Outlook 2003, Outlook will notify the user that a script is
trying to access a blocked property and prompt whether the access should be
allowed or not.
See http://www.slipstick.com/outlook/esecup.htm#autosec
And just to keep it confusing, whether you get a prompt depends on the context where the code is running and whether Exchange server-based security setings are in place.