Read Receipt Response Email displaying "Unknown"

[Gnanam]

I'm using Microsoft Office 2003 (Outlook), when I send email to more than 9
recipients in a single email addressed in "TO" field with "Read Receipt"
enabled, and if I receive Read Receipt email from all of the recipients, the
message shows only "Unknown" in the message body like the following:
_____________________________________________

Your message

To: Unknown
Subject:

_____________________________________________

Is there any option in Outlook that should be enabled before sending email,
or is there any Outlook updates / patch available to fix this problem?

Thanks in advance.

Regards,
Gnanam.