Cannot accept invitation from outside mail client

Hello all. I have an outside client who uses notes and sends an invitation to a meeting. When I accept the invitation, I get a delivery failure in the following format:

<outside_user/OU/Company@>@us.domain.com@mydomain

Error transferring to 10.55.23.137; SMTP Protocol Returned a Permanent Error 501 Bad address syntax

Would anyone happen to know how all of this gets appended to the mail message (invitation response) and what can be done to correct it? This user can send me an email to which I can repsond to with no problem. This only happens when I respond to meeting invitations.

Under document view for this invitation, the “Principle” field the information is listed as:

““outside user/OU/Company”<outside_user/OU/Company@>”

There is only one Principle field. I had a friend who also uses notes send an invitation as well. His message had two principle fields and one of the had his internet address in it. A response to his invitation went through. Any ideas?

Thanks