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.