Invitation accept from wrong person

Today, we have experienced a weird thing.

One of our users sent a meeting invitation to an internet recipient “xx.yy@zz.no”.

When receiving the accept message from the invitee, it looks as if the accept comes from a person in our Domino directory, named “xx aaaaaa/o” (he has the same first name as the invitee).

The SMPT originator field in the accept message shows as “xx.yy@zz.no”, and the $INetPrincipal field shows as "“xx"xx.yy@zz.no” (strange with the extra quotation marks around the first name). But the From and InviteeName fields shows as our internal guy, “xx aaaaaa/o”.

Inside the invitation, the response from the invited guys shows as “no response” and the internal guy is listed under “Unknown” with status Accepted.

Anyone who has any ideas? Is it an error in the Domino router, or is it an error in the invitee’s system?

Any help is appreciated.