Add an user property on Outlook 2003 mail item

I have developed an add in for Outlook 2003 which creates a custom field for a mail item using following code in C#:

Microsoft.Office.Interop.Outlook.UserProperty campo;

Campo=_oMailItem.UserProperties.Add("fldMatteo",Microsoft.Office.Interop.Outlook.OlUserPropertyType.olText, null, null);

campo.Value = "Example";

Then, once riceved the same mail to the destination Outlook, I was able to read the custom field with the following code:

Microsoft.Office.Interop.Outlook.UserProperty nuovocampo;

nuovocampo = m_oMailItem.UserProperties.Find("fldMatteo", true);

return "Valore: " + nuovocampo.Value;

Everithing was ok, it worked both with outlook client linked to an exchange server or simply using the outlook client to download the mail by pop3 protocol.

Once I upgraded the Office with KB913807 something has changed. Now I’m not more able to read this custom field if the client isn’t linked to an exchange server. If the client is linked to the exchange server everithing works.

How can I solve the trouble