Set filter to forward to another email and still deliver to user mailbox?

If it’s a Postfix thing, then what we need is the ability to execute a script as an action.

I coded one in PHP on another server that did exactly what you want to do, although that was just a requirement rather than the reason. Creating it from a script should avoid the error by remailing it from a different address like “robot@server.domain.tld” or something along those lines.

I wrote about it in this thread.

Richard