>GMail doesn't actually filter spam. It just hides it

This is not accurate. There is a long on-going issue with GMail (and others) wholesale blocking delivery of email from domains/IPs/providers it deems to be spam or otherwise inappropriate. GMail users get no indication/notification, and generally have no idea that email sent to them was simply not delivered.

Kind of. You can't turn their anti-spam off, you can't exclude addresses from it and, worse for me, Gmail can hard bounce if it doesn't like something in your server/IP/content in a way that's not fully documented or reproducible.

You can now. You can create a filter matching whatever criteria you want (including from:[email protected] or a catch all), and specify matches 'never get flagged as spam'. Its not obvious, but it is there. And the usual limitations of filters apply, such as not being able to match arbitrary headers.

That only works on mail that actually gets delivered to your mailbox. Gmail can and will:

- Reject delivery of mail from sources it has determined to be spammy (e.g. residential IPs) - this mechanism allows the sender to be aware that delivery failed.

- Accept delivery and then silently drop the message somewhere between after accepting delivery but before appearing in the end user's mailbox.

In either case, there is nothing you (the mailbox holder) can do. In the second case, there is no way for anyone to know the message hasn't been delivered.

Things can get flagged outside of your mailbox as so spammy that they will never arrive. This is how every spam filter works that I have seen to date and google is no exception.