Anything erroneously marked as spam can not be released to the forwarding address—-meaning they fail at their one job, forwarding email. Pobox had a great interface for quickly releasing messages to the forwarding address.
Pre-Fastmail, I did not have mail storage space at Pobox; just forwarding ability. I did not use Pobox's own interface for releasing spam mail; I used the standard filter (can't remember the exact name) and almost never saw nonspam in there (not that I checked often).
Post-Fastmail, I still forward from Pobox/Fastmail to the same other Google Workspace account from which I pull mail to my local system with `fetchmail`. I have Fastmail send all mail, spam or not; while the settings UI does not allow setting the spam protection level to "Off" when forwarding is used, the same thing can be achieved by using "Custom" then disabling "Move messages with a score of ___ or higher to Spam". I thus can let Google's spam filter deal with the inflow and, if necessary, manually sort miscategorized mail with my IMAP client.
