SMTP return address malformed!

I have a situation where meeting invites sent to people over the internet are getting a malformed reply address which causes replies to fail.

It seems to be limited to one user and only when his assistant sends it on behalf of him.

The meeting invite that the recpient out on the internet gets has a FROM field as follows:

From: jdoe@acme.com on behalf of Ron Smith/North/Acme <Ron_Smith/North/Acme@mail.acme.com

Notice that the first address is proper, but the second address has two odd things: it’s using his full notes name on the left of the @ and the mail.acme .com instead of only acme.com on the right of the @

We have the internet address field populated in the person documents and Lookups turned on in the server configuration, so based on this Ron’s address should be appearing as smithr@acme.com just like everyone else.

Ideas on where to look?

Lance