Within Teamwork Desk, you can enable Detect Manual Forwards which will allow Teamwork Desk to detect the original sender for any emails manually forwarded by an Agent or Administrator.
In order for this feature to work correctly, the email has to be to forwarded directly to Teamwork Desk from a Teamwork Desk user's email address without any alteration.
If the subject of the email is changed or text is entered in the body above the original email, Teamwork Desk may not detect that the email was forwarded by a Teamwork Desk user and post the user as the customer and not the actual sender of the original email.
To enable this, click on your profile icon to the top right of the Desk navigation bar and choose Settings from the dropdown.
In the settings area, select the Company tab.
From the message settings area you can then enable or disable detect manual forwards.
For more information see: Company Settings