Invalid Internet address specified

We have begun a migration to Exchange. We have an application which has a agent that sends emails to users in this format firstname.lastname@ourdomain.ca using the Domino SMTP server.

This has worked fine, but it is failing when it has to send to a user who has been migrated to Exchange. This message appears in the log:

Router: Error transferring message 0069D3DC via SMTP to Invalid Internet address specified

ip address is the address of the exchange server.

Messages to exchange users work fine when sent from iNotes, a notes client, or the outside world

We are running domino 6.5.4

Thanks for any help

Jim