As more mail servers use domain reputation to classify (accept/reject) incoming deliveries (helped with dmarc-like policies), messages from gmail.com will get a

As more mail servers use domain reputation to classify (accept/reject) incoming deliveries (helped with dmarc-like policies), messages from gmail.com will get a harder time getting delivered to mail servers they also try to deliver spam to. This provides natural pressure for gmail to reduce their outgoing spam rate. I'm sure they do try to get outgoing spam down. Spammers are just a resourceful bunch... Any large free email provider will have these problems. It's a reason to use your own domain for email, with your own reputation, instead of sharing your reputation with the whole world, including spammers.