I have an Outlook addin that creates UserProperties on MailItem objects. The addin has been working fine for about 6 years now. Recently I ran into an issue where a user of my addin is receiving the following error:
  • Exception: A custom field with this name but a different data type already exists. Enter a different name.


I am aware Outlook 2003 doesn't support OlUserPropertyType.olInteger and have code in place to use OlUserPropertyType.olNumber instead for Outlook 2003. I have a question out to the affected user to see if they upgraded from 2003 to 2007 or 2010 but are using the same *.pst file (which might pop in such a scenario) but haven't heard back yet.

The only other thing I can think of is an addin that competes for the same UserProperty name but a different data type. So I checked all the addins the user has and they only have one non-Microsoft addin. That being said, does anyone know which (if any) UserProperty names the "Outlook Change Notifier" addin by Apple creates/uses?