Just do a challenge-response on incoming mail from new addresses on Gmail and Outlook. Send an auto-reply telling them they can get their mail through if they copy a code in the reply. That will kill most spams from those sources.
It'd be nice if Google did something about this. Until they do, I tell everyone that uses Google for email that they have to accept that they're hosting with one of the biggest sources of spam on the Internet that, as far as I'm aware, does absolutely nothing when spam from them is forwarded to their abuse addresses.
Never auto-reply to any email ever. You're only making the spam problem worse.
(Plus, if you think there are not actual persons behind most Outlook/Gmail spam, I've got news for you. They will reply, and beg you for another chance, sometimes in highly emotional terms).
"550 5.7.1 The recipient has set a policy that prohibits email from this sender" at the SMTP level is the only way forward here.