Hi Patrick,

Thanks for reaching out. Can you please share details on how this forward is being done? Did you edit the user account and add a forwarding address, as shown here? 

If so, you might just need to adjust the "Keep original sender and recipient when forwarded" setting. 

When enabled, an email is essentially re-routed to the new recipient and keeps its original "To" and "From" addresses. When disabled, the forward behaves as if the email was manually forwarded, so the "From" address is replaced with the address of the mailbox forwarding the message, and the "To" address is the mailbox listed in the Forwarding Address.

I hope this helps!