in the expensive professional newsletter-systems I worked with, the problem is solved as follows:
forwarding means to the system that there is created a new newsletter without the actual secret tokens etc.
You also can’t forward the newsletter “normally” in your mail-client, but you have to click on a link in the newsletter (call it for example “forward to a friend”) – so you get to a page of the newsletter system, where you can write the email-address of the new user und your own (as sender. This is not a technically necessary feature, but for German law thus the legal responsibility lies with the sender ;)).
The forwarding-system of even the most professional newsletter-tools works more or less like a new subscription.
I hope this is of help 🙂
Best regards, connie