Exchange users send out meeting requests to Notes users. Notes users accepts it and the accepted notification comes back as an optional attendee with the original not accepting the meeting.
The initial meeting request is sent with an email address that is in the Notes person document. The InternetAddress of the Notes person document is not in the proxyAddress field of the AD/Exchange user. So when the Notes user accepts the request it goes back to the Outlook user but as a different user.
Confirm that the InternetAddress of the Notes person document is in the proxy address list of the AD/Exchange user.
