So it's not so simple. You could use DMARC to tell mail servers which honor DMARC to drop all e-mails that have @gmail.com addresses that don't come from gmail

So it's not so simple. You could use DMARC to tell mail servers which honor DMARC to drop all e-mails that have @gmail.com addresses that don't come from gmail.com servers. In fact, this is how @google.com e-mail addresses are treated, and I believe this is a setting which G-Suite administrators can set up for their domains. BUT. It comes with a downside. Suppose you want to send e-mail from your Linux laptop, or from a Linux mail server you control, without hard-coding your account password in a text file so you can send e-mail via a GMail server. Or suppose you want to subscribe to a mailing list which rewrites the subject line to include the mailing list name. DMARC breaks all of this. Horribly. So yes, it's more secure, but it comes with a massive cost. What this means, for example, is I recommend people who work at Google, and who want to interact with either IETF mailing lists or the Linux-kernel mailing lists at vger.kernel.org to send their patches and PULL requests using their @gmail.com address. If they send it using their @google.com address, the same security settings that will prevent this spammers from "faking" e-mails that didn't come from gmail servers, will also break git send-email (unless you want to save your password into at text file --- which is against policy and common sense) and it will break traditional mailing lists.