When forwarding email through the notes connector, each smtp address in the message header may have an @domainname appended as a suffix. This will subsequently break repliability to any of these messages once they are forwarded to exchange.
This issue is caused due to the way the Microsoft notes connector is configured.
Refer to Microsoft KB Article ID: 251955 - "Microsoft Exchange Connector for Lotus/Domino appends @SMTP@<Connector_for_Lotus_Notes_Domain> to the From address when receiving Internet mail for a Notes user" for an explaination of this issue and a resolution:
http://support.microsoft.com/kb/251955/EN-US/