From the SPR (SKJI7WMM4X) it is describe as:
"A user on the Internet sends a message to a Lotus Notes® user in your domain. The intended recipient of the message is defined in the Domino® Directory with a Person document specifying their mail system as "Other Internet Mail" with a
Forwarding Address populated. This forwarding address may be to a user outside of your Internet domain or to a user on another mail system internal to your domain. The message is properly received by this user at the address specified in
the Forwarding Address field in the Person document. However, when the recipient receives the message, it is addressed in the 'To' field to the recipient's original Internet address, not the forwarded address.
For example, if a message is sent to Joe Doe's Internet address of email@example.com and Joe has a Forwarding Address of firstname.lastname@example.org specified in his Person document, the message is forwarded, as configured. However, Joe Doe, when opening
the mail, sees email@example.com in the 'To' field, and not firstname.lastname@example.org.
If Joe Doe's mail system is Notes®, the 'To' field will be email@example.com, for mail coming from Notes or an alternate email application such as Microsoft® Outlook.
If the sender is a Notes user, the 'To' field will be firstname.lastname@example.org, no matter what Joe Doe's mail system is."