[xyzzy]

Originally Posted by projmanquest

I want to have a Fastmail email that forwards all incoming emails to another email address. But when I create a rule to "send a copy," it sends the email on as if it came from the original sender.]

So far so good - a "redirect a copy" rule can be created to do that. The "redirect" Sieve command can do that and that would be generated by the "redirect copy" Sieve filter option without you having to write your own explicit Sieve code.

I want it to arrive at the target email address as if it came from my Fastmail email address--just as if I manually forwarded it..

Now that's a problem. Don't see any way to do that, i.e. the part that says "...as if I manually forwarded it." I believe if you manually forward an incoming email it forwards the original just like the redirect command but I am not 100% certain of that.