Let's go through this one step at a time:
- Go to Settings>Spam Protection.
- Set the Protection level to Standard, Aggressive, or Custom. You must do this first.
- Go to Settings>Rules and click New Rule at the top.
- Click the link originally shown as "The sender's email" and change it to All messages.
- Click Send a copy to and enter the external email address in the box. For example, a Gmail address.
- You can also choose to discard the message so it won't be kept in your Fastmail account.
- Click Save at the top.
- You should now see a Forward rule of the form:
All messages > Send a copy to ....
- You can now click the big button in the upper left and return to the Mail screen. Send an email to yourself and you should see the forwarded copy within a few seconds in the external account. I just tested this with forwarding to Gmail.
Bill